WO2020013689A1

WO2020013689A1 - Active collimator system comprising a monolayer of monolithic converters

Info

Publication number: WO2020013689A1
Application number: PCT/NL2019/050426
Authority: WO
Inventors: Jeremy Michael Cooney BROWN; Dennis Robert Schaart
Original assignee: Technische Universiteit Delft
Priority date: 2018-07-13
Filing date: 2019-07-09
Publication date: 2020-01-16
Also published as: NL2021303B1

Abstract

The invention provides an active collimator system having a coded aperture, the active collimator system comprising (i) a monolayer of adjacently configured monolithic converters, wherein the monolithic converters are selected from the group consisting of scintillators and semiconductors, wherein the monolithic converters are configured to convert incident gamma rays and to provide a conversion product, and (ii) a sensor element configured to detect the conversion product; wherein the sensor element is configured at a surface of one or more of the monolithic converters; and wherein a set of two or more adjacently configured monolithic converters define a shared monolayer aperture, wherein the coded aperture comprises the shared monolayer aperture, wherein the shared monolayer aperture has an optical aperture axis O_a, wherein A_a is a smallest cross- sectional area of the monolayer aperture perpendicular to the optical aperture axis O_a, and wherein A_m is the largest cross-sectional area of the set of two or more adjacently configured monolithic converters with a plane perpendicular to the optical aperture axis O_a, and wherein A_a < 0.25 *A_m.

Description

Active collimator system comprising a monolayer of monolithic converters

FIELD OF THE INVENTION

The invention relates to an active collimator system. Further, the invention relates to a monolithic converter. The invention further relates to a nuclear imaging system comprising such active collimator. The invention yet further relates to the use of such active collimator or such nuclear imaging system for treatment delivery monitoring in radiotherapy treatment.

BACKGROUND OF THE INVENTION

Active collimators are known in the art. US8519343B 1 describes an apparatus for detecting and locating a source of gamma rays of energies ranging from 10-20 keV to several MeV's including plural gamma ray detectors arranged in a generally closed extended array so as to provide Compton scattering imaging and coded aperture imaging simultaneously. First detectors are arranged in a spaced manner about a surface defining the closed extended array which may be in the form of a circle, a sphere, a square, a pentagon or higher order polygon. Some of the gamma rays are absorbed by the first detectors closest to the gamma source in Compton scattering, while the photons that go unabsorbed by passing through gaps disposed between adjacent first detectors are incident upon second detectors disposed on the side farthest from the gamma ray source, where the first spaced detectors form a coded aperture array for two or three dimensional gamma ray source detection.

US2014091223A1 relates to a scintillation pixel array, a radiation sensing apparatus, a scintillation apparatus, and methods of making a scintillation pixel array wherein scintillation pixels have beveled surfaces and a reflective material around the beveled surfaces. The embodiments described herein may reduce the amount of cross-talk between adjacent scintillation pixels. GB2051 1 11A describes a scintillation detector comprising a right prism of bismuth germanate with one end in optical contact with a photo-multiplier and at least the side surfaces having a light reflective coating thereon e.g. painted white. The prism is preferably trapezoidal in one cross-section and optionally has its corners chamfered so that removeable heavy-metal plugs may be placed between adjacent prisms to give high spatial resolution. The chamfered corners increase efficiency to radiation received at angles up to 30°. The prisms and photo- multipliers are set on a common circle with heavy-metal spacers between the prisms and the front of the prisms are sealed with a foil of stainless steel. An array of detectors is used to detect coincident events in position annihilation tomography.

Lee and Lee, 2014,“Portable Active Collimation Imager Using URA Patterned Scintillator”, IEEE TRANSACTIONS ON NUCLEAR SCIENCE, describes an active collimator, wherein images of radiation passing through the holes of the collimator were reconstructed using the reconstruction methods of coded apertures, while the radiation scattered in the active collimator was imaged using a Compton imaging technique.

W02006120605A1 describes an imaging system comprising at least one radiation detector unit disposed adjacent a field of view to detect and measure radiation from the field of view. The detector unit includes multiple detection modules which each detect radiation from a prespecified region of the field of view, each region being a fraction of the field of view. One or more pinholes are associated with the detector unit. Each pinhole receives radiation from the prespecified region of the field of view and transmits radiation to one or more associated detection modules.

SUMMARY OF THE INVENTION

Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) systems are nuclear imaging systems. Nuclear imaging is a branch of nuclear medicine. Nuclear medicine, in general, concerns the use of radioactive compounds to diagnose and treat diseases. Nuclear imaging, specifically, is focused on detecting radiation from radioactive sources within a subject. The radioactive source is typically an administered radionuclide-carrying marker that targets a specific physiological process, resulting in local accumulations. These accumulations can then be imaged non-invasively as the radionuclides emit gamma rays. The gamma rays can be detected with a 2D scintigraphy system, but are most commonly detected using either PET or SPECT systems. In addition, PET and SPECT systems may also be combined with other imaging systems providing concurrent anatomical information. For example, the nuclear imaging systems PET and SPECT are combined with CT, and with MRI, resulting in PET-CT, PET-MRI, SPECT-CT, and SPECT-MRI hybrid imaging systems.

SPECT and PET are the two predominant in-vivo molecular imaging modalities for small animals and humans. In SPECT a molecular vector is labeled with a gamma ray emitting radionuclide, administered to the subject, and may be imaged via the use of a direct or coded aperture, composed of a highly attenuating material, to restrict the solid angle of radiation incident upon the surface of a position-resolving, or position-and-energy-resolving, radiation detector at some distance from the subject. For each detected gamma ray a Line of Response (LoR) that estimates its possible origin can be constructed utilizing the interaction location and estimated coded aperture opening it passed through. If the gamma ray could have passed through multiple coded aperture openings to reach the detected interaction location, either a single one must be chosen or a set of weighted LoRs are backprojected through each possible opening. For PET, a molecular vector is labeled with a positron emitting radionuclide and two co-linear 511 keV gamma rays are generated from its annihilation with an electron nearby the site of emission in the subject. Typically, the subject is placed into the center of a ring of position-and-energy-resolving radiation detectors configured to detect the pair of 511 keV gamma rays for each positron annihilation. When these two 511 keV gamma rays are detected within a pre-set time window, a LoR can be constructed for their interaction locations estimating the site of positron annihilation and the location of the molecular vector. For both molecular imaging modalities the backprojection of multiple LoRs enables for the distribution of the molecular vector within the subject to be estimated and with the aid of specialized image reconstruction programs quantitative estimates can be achieved.

SPECT and PET are powerful and commonly used imaging technologies in clinical and research settings. In these settings, a subject - a patient or animal - is administered a radionuclide marker that will concentrate in the body in a tissue-dependent manner, for example, cancer cells may become enriched with the marker. The marker will emit gamma rays of which the origin point can then be detected through nuclear imaging systems, such as SPECT or PET systems.

PET systems localize the origin point of a gamma ray by taking advantage of an annihilation event occurring when a positron encounters an electron; two 511 keV gamma rays are simultaneously emitted in roughly opposite directions. When both these gamma rays are detected by two different detector elements of the PET system, their origin point can be estimated as it has to roughly be on a line between the two detector elements.

In contrast, SPECT systems detect gamma rays without a paired gamma ray. In order to estimate the origin location of these gamma rays, SPECT systems rely on a collimator upstream of the gamma ray detectors. Each collimator aperture only permits gamma rays originating from a small area within the subject, thereby directly providing positional information for each detected gamma ray.

The incompatibility between PET and SPECT then arises from SPECT collimators needing to reject the vast majority of gamma rays in order to locate the emission origin, whereas PET detectors need to detect the vast majority of gamma rays as the localization depends on detecting two gamma rays from the same annihilation event. Yet, it may be of interest to combine SPECT and PET imaging systems to image SPECT and PET molecular vectors simultaneously.

A gamma ray is a type of photon. Gamma rays are defined according to two distinct definitions, both of which are used in the nuclear imaging field and herein. According to a first definition, gamma rays are the highest energy photons. Gamma rays have a partial overlap in the lower part of their energy spectrum with the highest energy (characteristic) X-rays, and gamma rays have no defined upper energy limit. This definition regards any photon with an energy above approximately 100 keV as a gamma ray. According to a second definition, a gamma ray is a photon emitted during the radioactive decay of an atomic nucleus from a high-energy state to a lower-energy state. This latter definition defines gamma rays by their origin irrespective of their energy. Following this definition, gamma rays do not have a defined energy range. Radionuclides used for SPECT emit gamma rays following the radioactive decay of an atomic nucleus. Such a gamma ray may have an energy below 100 keV, especially 70-90 keV, but also above 100 keV, especially 100-1000 keV, more especially gamma rays with an energy of 140 keV, 159 keV, 171 keV, 245 keV, 364 keV, 537 keV or 637 keV. Essentially all of the gamma rays emitted by major radiotracers, such as based on T1-201, Tc-99m, 1-123, 1-131, In-l l l, F-18, C-l l, etc., may be of interest. Hence, energies in the range of about 10 keV to 10 MeV may be of interest. Radionuclides used for PET can emit both gamma rays and positrons. When a positron encounters an electron an annihilation event, positron annihilation, takes place. In a positron annihilation event two 511 keV gamma rays are emitted in roughly opposite directions.

A characteristic X-ray is especially generated from the transition of an atomic bound electron to a lower, empty atomic state. These energies of these X-rays are dependent on the atom they come from and may be used in nuclear imaging systems, for example, in the case of iodine based radiotracers used in SPECT imaging. The term‘gamma ray’ herein relates to any photon that is used in nuclear imaging, irrespective of origin or energy (and also includes (characteristic) X-rays).

Gamma ray emission imaging is a widely applied technique composed of three primary methods of image formation (known as collimation): mechanical, coincidence/electronic, and Compton.

In mechanical collimation a gamma ray source, typically a radionuclide administered to a human or small animal, can be imaged via the use of a direct or coded aperture, typically composed of a highly attenuating material, to restrict the solid angle of radiation incident upon the surface of a position resolving, or position-and-energy-resolving, radiation detector at some distance from the gamma ray source. For each detected gamma ray a Line of Response (LoR) that estimates its origin can be constructed utilizing the interaction location and estimated aperture (“opening’') of the coded aperture it passed through. If the gamma ray could have passed through one of multiple apertures of the coded aperture to reach the detected interaction location, either a single one must be chosen or a set of weighted LoRs are projected through each possible opening.

In coincidence/electronic collimation a positron emitting source, again typically a radionuclide, provides two co-linear 51 1 keV gamma rays generated from the annihilation of an emitted positron with an electron nearby the site of emission. Typically the positron emitting source is located in the center of a ring of position-and-energy-resolving radiation detectors intended to detect the pair of 51 1 keV gamma rays for each positron annihilation. When these two 511 keV gamma-rays are detected within a pre-set time window, a LOR can be constructed spanning between their interaction locations estimating the site of positron annihilation and thereby the location of the emission by the positron emitting source.

Compton collimation is the process of reconstaicting the origin of a gamma ray through the use of Compton kinematics. Compton Imaging Systems (CISs) are typically comprised of either a single or a stack of position-and-energy- resolving radiation detectors. As the gamma ray travels through the active volume of the system, it may scatter a number of times and then possibly be photoelectrically absorbed. The energy, position and time stamps of these interactions - both the scattering interactions and the absorption interaction - can then be use to decode their order and to determine the incident gamma ray energy. At the site of the first interaction it is then possible to reconstruct a Cone of Response (CoR) to estimate the location of the gamma ray source with an axis given via the line between the first and second interaction location, and opening angle calculated using the Compton formula from the estimated incident gamma ray energy and the energy deposited at the first interaction site.

For all three collimation methods, backprojection of CoRs/LoRs enables estimating a probability distribution for the gamma ray source location and with the aid of specialized image reconstruction programs quantitative estimates can be obtained. The concept of active collimation enables e.g. simultaneous SPECT and PET molecular vector imaging with a reduced trade-off in performance. Instead of employing thick slabs of metal in the collimator to restrict the solid angle of radiation incident upon the surface of a position-resolving (or position- and-energy-resolving) radiation detector, gamma ray detectors are employed for this purpose instead. The coded collimator aperture limits the solid angle which emitted gamma rays from the SPECT molecular vector can pass through, thereby replicating the image formation process key to SPECT imaging. Because these collimators are active, the 51 1 keV gamma rays from the PET tracer that interact with them have their energy deposition, interaction location and time point recorded. A detected gamma ray can then be paired with its’ partner co-linear 51 1 keV gamma ray detected by another gamma ray detector to perform PET reconstruction. The result is a SPECT-PET molecular vector imaging system with near-zero loss of PET sensitivity whilst achieving similar SPECT resolution to a traditional metal collimator design.

Collimation systems known in the art may be expensive. Especially, the prior art systems may comprise expensive components. Applications, such as in homeland security, nuclear medicine (SPECT and PET), and radiotherapy dose delivery monitoring (prompt gamma-ray and active PET imaging in proton therapy), may be adequately addressed with a relatively easy solution.

Hence, it is an aspect of the invention to provide an alternative collimator system, and/or a nuclear imaging system, which preferably further at least partly obviates one or more of above-described drawbacks. The present invention may have as object to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

The present invention encompasses in embodiments the concept of single layered active collimation able to simultaneously image multiple gamma ray sources. Instead of employing a thick slab of metal to restrict the solid angle of radiation incident upon the surface of a position-resolving (or position-and- energy-resolving) radiation detector, a monolayer of monolithic converters comprising scintillators and/or semiconductors is utilized in isolation or integrated into the front of a larger collimation matrix (i.e. single layer of monolithic converters upstream of a coded thick metal collimator). This monolayer of monolithic converters enables for coded apertures to be constructed either in isolation or as part of a larger collimation matrix. Each of the monolithic converters may comprise a semiconductor or scintillator. Further, readout infrastructure (“sensor element”) may be arranged at one or more surfaces of a monolithic converter. In embodiments, the monolithic converters may be scintillators directly coupled to a spatially resolving digital photon counting photosensor unit.

Hence, in a first aspect the invention provides an active collimator system (“system”) having a coded aperture, the active collimator system comprising (i) a monolayer of adjacently configured monolithic converters (“converters”), wherein the monolithic converters are selected from the group consisting of scintillators and semiconductors, wherein the monolithic converters are configured to convert incident gamma rays and to provide a conversion product, and (ii) a sensor element configured to detect the conversion product; wherein the sensor element is configured at a surface of one or more of the monolithic converters, and wherein especially a set of two or more adjacently configured monolithic converters define a shared monolayer aperture (“external aperture”). The coded aperture comprises the shared monolayer aperture. In specific embodiments, the shared monolayer aperture has an optical aperture axis 0 , wherein A_a is a smallest cross-sectional area of the monolayer aperture perpendicular to the optical aperture axis O_a, and wherein A_m is the largest cross- sectional area of the set of two or more adjacently configured monolithic converters (with a plane) perpendicular to the optical aperture axis O_a. Especially, in specific embodiments A < l .00*A_m, like A_a £ 0.5*A_m, more especially A < 0.25*A_m, such as A_a < 0.25*A_m, especially A_a < 0.2*A_m, such as Aa < 0. l *A_m even more especially A_a < 0.05*A_m, such as A_a < 0.0l *A_m..

Amongst others, this invention may enable the simultaneous imaging of a subject using both SPECT and PET molecular vectors (SPECT-PET) with reduced drawbacks relative to current systems for SPECT-PET imaging. The active collimator system may serve as collimator for SPECT imaging, and at the same time the active collimator system may serve as gamma ray detector for PET imaging. Thereby, an encompassing hybrid SPECT-PET imaging system may have a combined SPECT-PET detector surface, rather than the separation of SPECT and PET detector surfaces found in alternative SPECT-PET systems. In addition, alternative SPECT and SPECT-PET systems may rely on metal collimators, which are typically incompatible with Magnetic Resonance Imaging (MRI). In contrast, the present invention may in embodiments also encompass MRI-compatible collimators. The present active collimator system may especially be useful for combined SPECT-PET systems, especially systems with additional imaging technologies such as with Computed Tomography (CT): SPECT-PET- CT; and with MRI: SPECT-PET-MRI.

The active collimator system according to the invention may be compatible with each of the three aforementioned primary methods of image formation (collimation): mechanical, coincidence/electronic, and Compton. Especially, during operation, each of the three image formation methods may beneficially be used simultaneously with the active collimator system as described herein.

Here below, the invention is described in more detail.

The active collimator system according to the invention may resemble an arrangement of monolithic converters in a grid, wherein each grid cell comprises a single monolithic converter, and wherein monolayer apertures are arranged along grid lines (“grid cell edges”). For example, in embodiments, the grid may be a grid of squares and the monolithic converters may approximate a rectangular cuboid shape, wherein the monolithic converters have, for example, shaved off corners along a height dimension. In such embodiments, monolayer apertures may be formed at the intersection of two grid lines, i.e., in such embodiment each set of four adjacently configured monolithic converters (also: “adjacent monolithic converters”) may define a shared monolayer aperture at their shared grid line intersection through having shaved off corners. In alternative or further embodiments, a monolithic converter may provide a cavity running along a surface along a height dimension of the monolithic converter, wherein the cavity may align with a similar cavity of an adjacently, especially adjoiningly, configured monolithic converter such that a shared monolayer aperture is formed in between the adjacently configured monolithic converters. In yet alternative or further embodiments, such cavity may be arranged against a flat surface of an adjacent monolithic converter, wherein the adjacent monolithic converter does not provide a cavity, still forming a shared monolayer aperture in between the adjacent monolithic converters.

The term“approximate” and its conjugations herein, such as in“to approximate a shape”, refers to being nearly identical to, especially identical to, the following term, for example nearly identical to a shape. For example, a cuboid monolithic converter having one or more cavities may approximate a cube, i.e., if it would not have the (small) cavities, it would be a cube. Hence, an object has a shape approximating a first shape. A first shape realization may be defined as the smallest encompassing shape of the (2D or 3D, respectively) object, wherein the first shape realization has the shape of the first shape, then a ratio of the area (volume) of the first shape realization to the area (volume) of the object is < 1.2, especially < 1.1, such as <1.05, especially <1.02. For instance, a cuboid monolithic converter with cavity (at e.g. a comer) has a shape approximating a cuboid shape. A first shape realization may be defined as the smallest encompassing cuboid (shape) of the (3D) cuboid monolithic converter with cavity, wherein the first shape realization has the shape of the cuboid (“first shape”), then a ratio of the volume of the first shape realization to the volume of the cuboid monolithic converter with cavity is < 1.2, especially < 1.1, such as <1.05, especially <1.02. The term“approximate” in the context of the elements of the invention will be clear to a person skilled in the art.

The active collimator system according to the invention may be (substantially) more flexible in the definition of apertures with respect to the prior art. Especially, the active collimator system may define apertures having different sizes and shapes, especially wherein the aperture sizes are (substantially) smaller than the size of a monolithic converter. The active collimator system according to the invention may in embodiments comprise but a single monolayer of monolithic converters, which may result in lower production costs with respect to systems relying on multiple layers of converters and/or pixelated converters. Despite the lower costs, the active collimator system may provide a moderate, or in specific embodiments high, resolution and sensitivity suitable for applications such as in homeland security, nuclear medicine (SPECT and PET), and radiotherapy dose delivery monitoring (prompt gamma-ray and active PET imaging in proton therapy).

The term‘‘monolayer aperture” refers to an aperture through the monolayer of adjacently configured monolithic converters. The term“shared” in shared monolayer aperture refers to the aperture being defined by two or more adjacently configured monolithic converters. The term“define” as in“adjacently configured monolithic converters define a shared monolayer aperture” refers to the monolithic converters marking the boundary of the shared monolayer aperture, i.e., the aperture comprises the space in between the surfaces of the adjacently configured monolithic converters. The shared monolayer aperture has an aperture edge, which is in embodiments defined by two or more adjacently configured monolithic converters. In embodiments, the monolayer aperture may have a shape selected from the group comprising a rectangular shape, a cylindrical shape, a (truncated) conical shape, or a (truncated) pyramidal shape.

The active collimator system may comprise a plurality of monolithic converters, e.g. a single active collimator system may include in the range of 2-5,000,000 monolithic converters, such as in the range of 3-5,000,000, especially in the range of 4-5,000,000, such as at least 16 monolithic converters, especially at least 32 monolithic converters, such as (at least) 64 monolithic converters. Hence, in further embodiments, the active collimator system comprises at least 2 monolithic converters, such as at least 3 monolithic converters, especially at least 4 monolithic converters, such as at least 16 monolithic converters, especially at least 32 monolithic converters, such as at least 64 monolithic converters.

In further embodiments, the monolithic converters may define the coded aperture. In further embodiments, the active collimator system may include in the range of 2-5,000,000 monolithic converters defining the coded aperture. Especially, the active collimator system may comprise at least 2 monolithic converters, wherein the at least 2 monolithic converters define the coded aperture. More especially, the active collimator system may comprise at least 3 monolithic converters, wherein the at least 3 monolithic converters define the coded aperture, such as at least 4 monolithic converters, such as at least 16 monolithic converters, especially at least 32 monolithic converters, such as at least 64 monolithic converters.

A collimator is a device configured to control one or more of beam (of rays) direction, beam (of rays) width, and beam (of rays) path. The collimator comprises one or more apertures through which a gamma ray can pass, and it comprises a collimator material configured to absorb gamma rays. The one or more apertures are configured to provide desired paths stretching between two opposite sides of the collimator along which a gamma ray can travel without encountering collimator material. A gamma ray travelling along a desired path may pass the aperture, such as a path coinciding with an optical axis of the collimator. A gamma ray not travelling along a desired path may not pass the collimator, and may be absorbed by the collimator material. The collimator is configured to filter gamma rays such that the outgoing beam of gamma rays are more focused or narrowed, especially such that only gamma rays with desired incident angles are accepted, more especially such that the directions of the outgoing gamma rays are more parallel than upstream of the collimator.

The active collimator system has a coded aperture. The coded aperture is defined by one or more apertures, especially by two or more apertures. Especially, the coded aperture may comprise at least 2 apertures, such as at least 3 apertures, especially at least 4 apertures, such as at least 6 apertures, especially at least 8 apertures, such as at least 16 apertures. The one or more apertures are defined by the monolithic converters. Hence, the active collimator system may have a coded aperture face, which coded aperture face essentially defines the coded aperture. Thus, the coded aperture face is essentially defined by the monolithic converters, more especially by surfaces of the monolithic converter (see also below). Hence, the monolithic converters of the active collimator system are configured in such a way, that a collimator with a coded aperture is provided (thereby). Coded apertures or coded-aperture masks may be grids, gratings, or other patterns, typically of materials opaque to various wavelengths of light. The wavelengths are usually high-energy radiation such as gamma rays. By blocking light in a known pattern, a coded "shadow" is cast upon a plane. The properties of the original light sources can then be mathematically reconstructed from this shadow. Coded apertures are used in gamma ray imaging systems, because these high-energy rays cannot be focused with lenses or mirrors. Herein, the term “coded” or“coded aperture” and similar terms especially refer to the aperture that is defined by the monolithic converters. The tenn“coded aperture” may (thus) refer to an aperture defined by adjacently configured monolithic converters but may also refer to an arrangement of a plurality of apertures (defined by the monolayer of adjacently configured monolithic converters).

The conversion product provided by a monolithic converter depends on the monolithic converter, i.e., a scintillator may convert a gamma ray to scintillation photons (as conversion product(s), whereas a semiconductor may convert a gamma ray to an electric current (as conversion product).

The sensor element is configured to detect the conversion product. Hence, a sensor element functionally coupled to a scintillator may be configured to detect scintillation photons. Similarly, a sensor element functionally coupled to a semiconductor may be configured to detect an electric current. The sensor element may especially be configured to record one or more of a time, location, and energy (transfer) of an interaction, i.e., one or more of a time, location and energy of an interaction between a gamma ray and a (respective) monolithic converter. In embodiments, a sensor element may be functionally coupled to a plurality of monolithic converters, especially to a plurality of monolithic converters of the set of two or more adjacently configured monolithic converters. In alternative and or further embodiments, a sensor element may be functionally coupled to one monolithic converter. Hence, in embodiments, the active collimator system may comprise a plurality of sensor elements, wherein each of the sensor elements is functionally coupled to one monolithic converter, and wherein each of the monolithic converters is functionally coupled to one of the sensor elements. The sensor element may be configured at a surface of one or more of the monolithic converters, especially at a part of the surface of one or more of the monolithic converters, especially at a part of the surface of one or more of the monolithic converters of the set of two or more adjacently configured monolithic converters, i.e., in embodiments the sensor element may not cover the complete surface of one or more of the monolithic converters. In alternative or further embodiments, the sensor element may be configured on a surface of one or more of the monolithic converters, especially on a part of the surface of one or more of the monolithic converters. Hence, in embodiments, a sensor element may be configured at a (top or bottom or side) surface of a single monolithic converter.

The phrase“at a surface” as in“configured at a surface” herein especially refers to a location on or proximal to such surface. Hence, the sensor element may be on a surface of one or more of the monolithic converters, i.e., may directly be in contact with the surface. Alternatively, the sensor element may be configured at a small distance from a surface of one or more of the monolithic converters, such as configured at up to 5 cm, such as up to 2 cm, especially up to 1 cm, such as up to 5 mm, especially up to 2 mm from the surface(s) of one or more of the monolithic converters.

In a further or alternative embodiment, a sensor element may be configured in between adjacently configured monolithic converters of a set of two or more adjacently configured monolithic converters, i.e., configured at side surfaces of two or more adjacently configured monolithic converters, wherein the sensor element may be functionally coupled to one or more of the adjacently configured monolithic converters. In general, a sensor element bordering a monolithic converter is functionally coupled to the respective monolithic converter. In yet further or alternative embodiments, a sensor element may be configured at a plurality of top or bottom surfaces of different monolithic converters.

In embodiments, a sensor element may cover part of a surface of a monolithic converter. In general, a sensor element may fully cover a surface of a monolithic converter. Each aperture has an optical aperture axis O_a, wherein the angle and location of the optical aperture axis coincide with a weighted average of the possible paths along which light can pass through the aperture. In embodiments, the optical aperture axes of different apertures may be parallel, especially the optical aperture axes of different apertures may be configured at an angle, i.e., they may be non-parallel.

In embodiments, the size of a monolayer aperture may be substantially smaller than the size of an adjacent monolithic converter. Especially the cross-sectional area through which a gamma ray may pass is substantially smaller than the cross-sectional area through which the gamma ray may encounter a monolithic converter with respect to the optical aperture axis O_a of the monolayer aperture.

Especially, perpendicular to an optical aperture axis O_a of the monolayer aperture, the smallest cross-sectional area of the monolayer aperture (A_a) may be substantially smaller than the largest cross-sectional area of the set of two or more corresponding adjacent monolithic converters (A_m) with a plane (also: “in a plane”) perpendicular to the optical aperture axis O_a, such as A_a < 0.5*A_m, especially A_a < 0.2*A_m, such as A_a < 0. l *A_m, more especially A·, < 0.05*A_m. The phrase“cross-sectional area with a plane (also:“in a plane”) perpendicular to the optical aperture axis O_a” indicates that A_m refers to the cross-sectional area of the set of two or more adjacently configured monolithic converters with a plane, wherein the plane is perpendicular to the optical aperture axis O_a, i.e. A_m only comprises the area covered by one of the monolithic converters, not the area comprised by an aperture. Hence, A_m may be a summation of the cross-sections of the individual adjacently configured monolithic converters (defining the aperture) with the (same) (virtual) plane.

Especially, the active collimator system may comprise a plurality of monolayer apertures wherein in average the smallest cross-sectional area of each of the monolayer apertures (A_a) is substantially smaller than the (respective) largest cross-sectional area (A_m) (in a plane perpendicular to the optical aperture axis O_a) of the set of two or more corresponding adjacently configured monolithic converters (that define the monolayer aperture), such as A_a < 0.5*A_m, especially Aa < 0.2*A_m, such as A £ 0. l *A_m, more especially A_a < 0.05* A,,,.

Even more especially, the active collimator system may comprise a plurality of monolayer apertures wherein the smallest cross-sectional area of all of the monolayer apertures (A_a) is substantially smaller than the largest cross- sectional area of the two or more corresponding adjacent monolithic converters (A_m) (in a plane perpendicular to the optical aperture axis O_a), such as A £ 0.5* Am, especially A_a < 0.2*A , such as A_a < 0.1 *A_m, more especially A_a < 0.05*A_m, (that define the respective monolayer apertures, i.e. the coded aperture).

Note that a single monolithic converter may be configured to define at least parts of two or more monolayer apertures.

The cross-sectional area of the adjacent monolithic converters A_m only comprises the area covered by the set of adjacently configured monolithic converters, i.e., the area of any (other) aperture is not comprised in A_m. In general, the smallest cross-sectional area of the monolayer aperture may lie in the same plane as the largest cross-sectional area of the two or more corresponding adjacent monolithic converters (in a plane perpendicular to an optical aperture axis O_a of the monolayer aperture). However, in embodiments, the smallest cross-sectional area of the monolayer aperture may lie in a different plane than the largest cross- sectional area of the two or more corresponding adjacent monolithic converters (in a plane perpendicular to an optical aperture axis O_a of the monolayer aperture). For example, one or more of (i) other apertures, (ii) slanted monolithic converter surfaces, and (iii) angles between adjacent monolithic converters may result in the smallest cross-sectional area of the monolayer lying in a different plane than the largest cross-sectional area of the two or more corresponding adjacent monolithic converters.

As A_a refers to the cross-sectional area of a monolayer aperture (in a plane perpendicular to the optical aperture axis O_a), A is non-zero, i.e., A > 0 mm², such as A ³ 0.0001 mm². Especially, the cross-sectional area of each monolayer aperture A is non-zero, i.e., A·, > 0 mm², such as A_a > 0.0001 mm². Especially, A_a may have a minimal size relative to A_m, such as A ³ 0.000 l *A_m, especially A ³ 0.00l *A_m, such as A ³ 0.005*A_m, more especially A ³ 0.0l *A_m. In further specific embodiments, A_a > 0. l *A_m. Especially, in embodiments A_a < 0.25 A_m, such as such as A·, < 0.25*A_m.

In yet specific embodiments, the cross-sectional area of an aperture A (in a plane perpendicular to the optical aperture axis O_a) may have a similar size to the largest cross-sectional area of one of the monolithic converters of the two or more corresponding adjacent monolithic converters (part of A_m) (in a plane perpendicular to the optical aperture axis O_a). Therefore, in embodiments, 0. l l *Am£ Aa £ 1.5* Am, such as 0. l l *A_m £ Aa < 1.0* A_m, especially 0.2*A_m £ A_a < 0.5* A_m. For example, for an aperture having the same size as a monolithic converter in a regular square grid, A_a ~ 0.25 *A_m (see fig. 8 A). Hence, in specific embodiments, A_a > 0.25 *A_m. Especially, the active collimator system comprises in embodiments a plurality of apertures, wherein the cross-sectional area of each aperture A_a (in a plane perpendicular to the optical aperture axis O_a) has a similar size to the largest cross-sectional area of the two or more corresponding adjacent monolithic converters (A_m) (in a plane perpendicular to the optical aperture axis Oa), such as 0.1 1 *A_m< Aa < 1.5* Am, especially 0.1 l *A_m £ Aa < l .00*A_m.

Note that in a purely cubic arrangement of cuboid monolithic converters, in a 3x3 grid, with the center cell of the grid configured as aperture (thus eight monolithic converters), effectively only four monolithic converters may define the aperture. Likewise, in a checkerboard grid effectively only four monolithic converters may define the aperture.

In other embodiments (however), the cross-sectional area of an aperture A_a (in a plane perpendicular to the optical aperture axis O_a) is equal to or smaller than each of the largest cross-sectional areas of the individual monolithic converters defining the aperture. The largest cross-sectional areas of each of the individual monolithic converters may be indicated as A_ma . The largest cross- sectional areas of the monolithic converters A_max may differ for the different monolithic converters defining the aperture, though in general they will be the same. Especially, in embodiments A_a < A_max.

In specific embodiments, A_a = A_max. In further specific embodiments, A_a < A_max. In further specific embodiments, A_a > 0.25 *A_m. In further or alternative specific embodiments, A_a < 0.25 *A_m. In other specific embodiments, Aa < 0. l*Am x. Especially, A_a < 0. l*A_m. For instance, 0.000 l*A_m < Aa < 0. l *A_m.

Especially, the monolithic converters are selected from the group consisting of scintillators and semiconductors. Hence, the active collimator system may comprise both scintillators and semiconductors. Especially, the active collimator system may comprise semiconductor scintillators, i.e., semiconductors that are also scintillators. In general, either all monolithic converters are scintillators, or all monolithic converters are semiconductors.

The monolithic converter(s) may have a shape approximating a right regular prism, especially wherein the bases are regular polygons selected from the group comprising regular triangles, regular squares and regular hexagons. The phrase“approximating a right regular prism” indicates that the monolithic converters may resemble such shape but may have one or more sections removed. For example, in embodiments the monolithic converters may resemble a right regular prism having a regular square base (top and bottom surface) except that the monolithic converters lack one or more corners of the prism all along a height dimension of the monolithic converters. Hence, the monolithic converters may approximate a shape of a right regular prism, especially the top and/or bottom surface may approximate a regular polygon, such that the monolithic converters may be arranged in approximately a tessellating grid but for the apertures that are formed in between two monolithic converters at those locations where the monolithic converters deviate from the shape of the right regular prism. Hence, in embodiments monolithic converter(s) may have a shape approximating a cuboid.

In embodiments, the monolithic converters may have been first produced as right regular prisms but one or more sections of the monolithic converters may subsequently have been shaved off, or cut away, via one or more of laser cutting, water jet cutting and diamond edge cutting. Hence, in embodiments, first a monolithic converter having a right regular prismatic shape is produced, wherein after production of the monolithic converter one or more sections of the monolithic converter are removed (thereby providing cavities) via one or more of laser cutting, water jet cutting and diamond edge cutting. Especially, at least part of one or more edges of the monolithic converter having a right regular prismatic shape may be removed.

In embodiments, (all) different monolithic converters may have the same shape, especially different monolithic converters may have different shapes; more especially, different monolithic converters may approximate different right regular prisms.

In embodiments, the monolithic converters may have dimensions like a height selected from a few millimeters to centimeters, and a width and depth selected of similar range. In embodiments, the width and depth may be constant over the height, especially, the width and depth may vary over the height. Different monolithic converters may have the same size, especially different monolithic converters may have different sizes. In embodiments, each monolithic converter may have a size selected from the range of 2 x 2 x 2 mm - 48 x 48 x 48 mm, especially from the range of 4 x 4 x 4 mm - 48 x 48 x 48 mm, more especially from the range of 4 x 4 x 4 mm - 36 x 36 x 36 mm, such as 32 x 32 x 20 mm. In general, all monolithic converters may have a substantially identical size, such as an identical size. The term“2 x 2 x 2 mm - 48 x 48 x 48 mm” and similar terms refer to that any of the three orthogonal defined sizes may (independently) be selected from the range of 2-48 mm (respectively).

The coded aperture may comprise a plurality of apertures, especially a plurality of shared monolayer apertures, more especially a plurality of shared monolayer apertures and one or more interior monolayer apertures. The term“aperture” herein refers to an opening stretching from one side (for example, the top side) of the monolayer to the other side (the bottom side) of the monolayer such that a gamma ray may pass through the aperture in the monolayer without encountering a monolithic converter. In embodiments, the active collimator system may be configured such that - during operation - the top surfaces of the monolithic converters face a gamma ray source location. In embodiments, the top surface and the bottom surface of a monolithic converter may be substantially identical, especially, the top surface and the bottom surface may be (approximate) mirror images (of each other). Hence, the top surface and the bottom surface of a monolithic converter may be substantially identical, but herein the term“top surface” may especially be used to indicate the surface that would face a gamma ray source location during operation. The term“shared aperture” refers to an aperture that is defined by two or more adjacent monolithic converters, i.e., borders two or more adjacent monolithic converters. In contrast, the term“interior aperture” refers to an aperture that is defined by a single monolithic converter. The term“aperture” may also refer to a plurality of apertures. The active collimator system as described herein may include a single aperture but may in other embodiments comprise a plurality of apertures.

The coded aperture is configured to selectively accept gamma rays, i.e., to selectively allow gamma rays to pass, especially the coded aperture may accept gamma rays based on their incidence angle and location. In embodiments, the shape of the coded aperture may approximate the shape of one or more of a parallel hole collimator, a slant hole collimator, a converging collimator, a diverging collimator, a fan beam collimator, a single pass diverging collimator, a pinhole collimator, or any other shape resulting in the collimation of gamma rays, especially one or more of a parallel-hole shape, a pinhole shape, a converging shape, or a diverging shape. As indicated above, the coded aperture may comprise a single aperture or may comprise a plurality of apertures. In the latter embodiment, the apertures may have essentially identical dimensions, but may also have different dimensions.

In embodiments, the coded aperture may have one or more of a parallel-hole shape, a pinhole shape, a converging shape, or a diverging shape. The term“shape” may also indicate that the coded aperture approximates such shape, as will be clear to a person skilled in the art. Therefore, in embodiments the coded aperture approximates the shape of a parallel-hole shape, of a pinhole shape, of a converging shape, or of a diverging shape. In specific embodiments, a combination of two or more different types of aperture shape may be applied (within the same active collimator system).

In embodiments, a coded aperture having a pinhole shape may comprise one or more monolayer apertures. In further or alternative embodiments, a coded aperture having one or more of a parallel-hole shape, a converging shape, or a diverging shape comprises a plurality of monolayer apertures. In embodiments, the active collimator system may further comprise a metal insert arranged between two or more adjacent monolithic converters to further define a shared monolayer aperture. Hence, an aperture defined by adjacent monolithic converters may be further defined by a metal insert. The metal insert may further define the shared monolayer aperture by one or more of narrowing, widening, focusing, and/or slanting the aperture. The metal insert may partially cover a surface of a monolithic converter. Especially, the metal insert may fully cover a surface of a monolithic converter. The metal insert may be an (elongated) plate. Alternatively, a metal insert may be hollow, having cross-sections e.g. selected from the circular, square, hexagonal, etc. cross-sections. The metal insert may have a variable cross-section (along an optical aperture axis of the shared monolayer aperture). Especially, the metal insert may have a static cross-section. Different metal inserts may all have the same shape. Especially, different metal inserts may have different shapes. Especially suitable materials have a high density, are non-toxic, and are stable in air. The metal insert may especially comprise one or more of platinum, tungsten, lead, gold, iridium, rhenium, tantalum, rhodium, ruthenium and molybdenum, especially platinum, tungsten, lead, molybdenum and gold, more especially two or more of platinum, tungsten, lead, molybdenum and gold, e.g. the metal inserts may comprise both tungsten and gold. Hence, the shape of the coded aperture may be further defined through a metal insert, especially through a plurality of metal inserts.

A gamma ray may penetrate solid materials, i.e. pass through solid materials without being absorbed, especially a high energy gamma ray may penetrate the collimator material. Penetration of the collimator material is undesired as a gamma ray not travelling along a desired path may pass the collimator and appear accepted. Therefore, the collimator may be configured to decrease the likelihood of penetration. The likelihood of penetration may be reduced by the choice of collimator material and by increasing the thickness of the collimator material. Hence, in embodiments, the monolithic converters may have a thickness, especially height, selected to reduce the likelihood of penetration of gamma rays. Especially, the height of the monolithic converters may be selected such that the likelihood of gamma rays having a first energy penetrating a monolithic converter is in the range of 0-10%, such up to 5%, like in the range of 1-5%, wherein the first energy may especially be selected from the range of 100- 1000 keY, especially the first energy may be selected from the group consisting of 140 keY, 159 keV, 171 keV, 245 keV, 364 keV, 537 keV and 637 keY. In embodiments, the height of the monolithic converter may be selected such that a gamma ray having an energy of 140 keV (incident on the monolithic converter(s)), and in embodiments e.g. originating from a radiotracer used for SPECT imaging, has a low likelihood of penetration, such as a likelihood of penetration selected from the range of 0-5%. In such embodiments, a gamma ray having an energy of 511 keV and originating from a radiotracer used for PET imaging may have a higher likelihood of penetration, such as a likelihood of penetration exceeding 5%. Penetration may be less problematic for image reconstruction in PET imaging than for SPECT imaging. In specific embodiments, the active collimator system may be configured such that the thickness (height) is specifically suitable for the collimation (low rate of penetration) of gamma rays having a first energy, wherein the first energy is selected from the group comprising 140 keV, 159 keV, 171 keV, 245 keV, 364 keV, 537 keV and 637 keV, especially from the group consisting of 140 keV, 159 keV, 171 keV, 245 keV, 364 keV, such as from the group consisting of 140 keV, 159 keV. By specifically designing the active collimator system for gamma rays having a relatively low energy, less material may be needed for the monolithic converters, thereby reducing costs of the active collimator system at the expense of versatility in applications. Hence, in embodiments, the thickness of the active collimator system, especially the height of the monolithic converters, may be selected such that the penetration of gamma rays (incident on the monolithic converter(s)) having a first energy <5%, wherein the first energy is selected from the group comprising 140 keV, 159 keV, 171 keV, 245 keV, 364 keV, 537 keV and 637 keV. The thickness of the collimator material, especially scintillating or semiconductor material, required to limit the penetration of gamma rays to 10%, especially to 5% depends at least on (i) the gamma ray energy, (ii) the collimator material, and (iii) the coded aperture shape. In embodiments, an active collimator system comprising 1 cm of LYSO may, for example, prevent penetration of at least 95% of gamma rays having an energy selected from the group consisting of 140 keV and 159 keY. It will be clear to a person skilled in the art what thickness (height) has to be selected to provide the low rate of penetration depending on the collimator shape and material.

In alternative embodiments, the active collimator system may further comprise a metal collimator configured to reduce the likelihood of penetration. Hence, the active collimator system may further comprise a metal collimator comprising a second coded aperture, wherein the second coded aperture comprises a metal collimator aperture, wherein the metal collimator is arranged such that the metal collimator aperture is aligned with the shared monolayer aperture of the set of two or more adjacently configured monolithic converters. Especially, the metal collimator may be arranged adjacent to the monolayer of adjacently configured monolithic converters. In general, each monolayer aperture of the coded aperture may transition into a metal collimator aperture of the metal collimator, i.e., each monolayer aperture of the coded aperture may be arranged aligned with a corresponding metal collimator aperture of the second coded aperture, especially wherein the optical aperture axis of the monolayer aperture of the coded aperture is (substantially) identical to the optical aperture axis of the metal collimator aperture of the second coded aperture. The collimator material in prior art solutions and in the metal collimator may comprise an attenuating material, especially it may comprise a material with a high attenuation coefficient. Materials with a high attenuating coefficient may be selected from the group comprising platinum, tungsten, lead, gold, iridium, rhenium, tantalum, rhodium, ruthenium and molybdenum. Hence, in embodiments, the metal collimator may comprise one or more of platinum, tungsten, lead, gold, iridium, rhenium, tantalum, rhodium, ruthenium and molybdenum, especially one or more of lead, gold, tungsten, molybdenum, or platinum.

In embodiments, the metal collimator is configured to support the monolayer of adjacently configured monolithic converters. Hence, a (collimator) stack of a metal collimator and the monolayer of adjacently configured monolithic converters may be provided.

Further, in alternative embodiments the active collimator system exclusively comprises a non-magnetic material. An advantage of an active collimator system exclusively comprising non-magnetic materials may be its compatibility with MRI and/or other anatomical and/or functional imaging systems.

In embodiments, a set of two adjacent monolithic converters may be arranged at an angle a, wherein 45° < a < 180°, such as 90° < a < 180°, especially 100° < a < 180°, more especially wherein 135° < a < 180°, such as 150° < a < 175°, i.e., the virtual planes perpendicular to the height dimensions of the adjacently configured monolithic converters may be arranged at an angle a, wherein 45° < a < 180°, such as 90° < a < 180°, especially 100° < a < 180°, more especially wherein 135° < a < 180°, such as 150° < a < 175°. In further embodiments, a set of two adjacent monolithic converters may be arranged at an angle a, wherein 45° < a < 175°, such as 90° < a < 175°, especially 100° < a < 175°, more especially wherein 135° < a < 175°, Especially, a row of adjacent monolithic converters may be arranged at an angle with an adjacent row of adjacent monolithic converters. More especially, each row of adjacent monolithic converters (along one or more dimensions) may be arranged at an angle with an adjacent row of adjacent monolithic converters. In specific embodiments, at least two adjacently configured monolithic converters may be arranged at an angle a < 180°, such as <175°, especially < 150°.

Such an embodiment may be beneficial when a sensor element is configured in between two adjacent monolithic converters. For example, the angle may be selected such that the adjacent monolithic converters touch despite the sensor element being configured in between. For example, edges bordering the top surfaces of two adjacent monolithic converters may touch, whereas the sensor element is configured at the side surfaces near the edges bordering the bottom surfaces of the respective two adjacent monolithic converters.

Further, such embodiment may be beneficial to provide a curved surface for the active collimator system. For example, PET detectors may be beneficially placed in a ring surrounding a subject.

In specific embodiments, the angle between adjacent monolithic converters may be flexible. The flexible angle may enable to position the active collimator system in a beneficial position surrounding a subject, especially surrounding a specific body part of a subject, such as surrounding the neck of a subject.

In embodiments, at least one of the monolithic converters of a set of two or more monolithic converters may comprise a slanted surface, wherein the slanted surface defines at least part of the shared monolayer aperture. The term “slanted surface” refers to a surface that is not parallel or perpendicular to a top (or bottom) surface of a monolithic converter. A slanted surface bordering a shared monolayer aperture may be beneficial to adjust the cross-section of the monolayer aperture perpendicular to an optical aperture axis of the monolayer aperture. For example, a monolayer aperture bordering multiple slanted edges may approximate the shape of a pinhole aperture, especially, it may have the shape of a pinhole aperture.

In specific embodiments, the slanted surface of a monolithic converter may be configured such that no aperture would be formed if the slanted surface is placed against a flat surface of an adjacent monolithic converter, but that a slanted monolayer aperture would be formed if the slanted surface were placed against a similarly slanted surface of an adjacent monolithic converter, for example wherein one of the monolithic converters comprises a slanted surface bordering the top surface, while the adjacent monolithic converter comprises a slanted surface bordering the respective bottom surface. The slanted monolayer aperture may run diagonally with respect to the adjacent monolithic converters. Such slanted aperture may be especially suitable for an active collimator system wherein the coded aperture approximates a converging shape or a diverging shape.

In embodiments, a set of two adjacently configured monolithic converters may define at least one shared monolayer aperture of the coded aperture. In further embodiments, the at least one shared monolayer aperture of the coded aperture is formed by adjacently configured cavities in both adjacently configured monolithic converters of the set, i.e., two cavities of two adjacently configured monolithic converters are aligned and form a single shared monolayer aperture. In an alternative embodiment, the at least one shared monolayer aperture of the coded aperture may be formed by a single cavity in one of the adjacently configured monolithic converters of the set. Shared monolayer apertures may be defined both by aligned cavities as well as by single cavities in the same embodiment.

In alternative or further embodiments, a set of three or more adjacently configured monolithic converters defines at least one shared monolayer aperture of the coded aperture. Especially, three or more adjacently configured monolithic converters meet at an intersection (point) bordering all of the three or more adjacently configured monolithic converters, wherein the three or more adjacently configured monolithic converters define a shared monolayer aperture at the intersection point. In a further embodiment, the at least one shared monolayer aperture of the coded aperture may be formed by adjacently configured cavities in all of the three or more adjacently configured monolithic converters of the set.

In specific embodiments, the adjacently configured monolithic converters are arranged in a grid (with the grid) having a plurality of rectangular, such as square, grid cells and grid cell lines, wherein each of the grid cells comprises a single monolithic converter having a cuboid shape, wherein intersecting grid cell lines define an intersection, and wherein a set of four adjacently configured monolithic converters define a shared monolayer aperture at the intersection. Especially, in embodiments each of the four adjacently configured monolithic converters comprise a slanted face facing the intersection, wherein the slanted face of each of the four adjacently configured monolithic converters defines at least part of the shared monolayer aperture.

In embodiments, the active collimator system may comprise one or more of the aforementioned sets of two adjacently configured monolithic converters and one or more of the aforementioned sets of three or more adjacently configured monolithic converters.

The active collimator system comprises a monolayer of adjacently configured monolithic converters, especially a monolayer of adjoiningly configured monolithic converters. The term“monolayer” refers to the monolithic converters being arranged in a side-by-side manner, i.e., they are especially not (3D) stacked. Hence, a monolithic converter may have a top surface, a bottom surface, and a plurality of side surfaces, wherein the side surfaces may face (one or more) adjacent monolithic converters. Hence, in embodiments the active collimator system comprises a 2D array of adjacently configured monolithic converters (optionally supplemented with a metal collimator (see also above)).

Each monolithic converter has an independently selected height H (also“thickness”) defined by the (shortest) distance between the top and the bottom surface. In general, the top and bottom surface of the monolithic converters are parallel. In embodiments wherein the adjacent monolithic converters are configured at an angle a, the top and bottom surface of the respective monolithic converters may not be parallel, especially, the top and bottom surfaces of the respective monolithic converters may be arranged at the angle a. In embodiments, two or more monolithic converters may have different heights. Especially, however, two or more, especially all, monolithic converters may have the same height.

In further or alternative embodiments, the monolithic converter(s) may be slab-shaped, i.e., the height of the monolithic converter may be smaller than its length in one or more other dimensions perpendicular to its height (such as a width and depth), especially the height H of a monolithic converter may be smaller than any other (monolith) length L_mx of the monolithic converter defined by other parallel surfaces of the monolithic converter. Hence, in embodiments, each monolithic converter may have a height H, especially wherein H is smaller than the longest length of the respective monolithic converter in each dimension perpendicular to H, more especially wherein each monolithic converter has the same height H. The slab-shaped monolithic converters may be configured in a layer, with essentially height H, wherein the height H of a monolithic converter may be smaller than any other (monolith) length L_mx of the monolithic converter defined by other parallel surfaces of the monolithic converter.

In alternative embodiments, the monolithic converter(s) may be bar-shaped, i.e., the height of the monolithic converter may be larger than its length in one or more other dimensions perpendicular to its height (such as a width and depth), especially the height H of a monolithic converter may be larger than any other (monolith) length L_mx of the monolithic converter defined by other parallel surfaces of the monolithic converter. Hence, in embodiments, each monolithic converter may have a height H, especially wherein H is larger than the longest length of the respective monolithic converter in each dimension perpendicular to H, more especially wherein each monolithic converter has the same height H. The bar-shaped monolithic converters may be configured in a layer, with essentially height H, wherein the height H of a monolithic converter may be larger than any other (monolith) length L_mx of the monolithic converter defined by other parallel surfaces of the monolithic converter.

Each monolithic converter may further have virtual planes perpendicular to its height. In embodiments, the virtual planes of (all) adjacently configured monolithic converters may be parallel i.e., the virtual planes of the adjacently configured monolithic converters may be arranged at an angle of 180°). In alternative embodiments, the virtual planes of a set of two or more adjacently configured monolithic converters may be arranged at an angle a, wherein 45° < a < 180°, such as 90° < a < 180°, especially 100° < a < 180°, more especially wherein 135° < a < 180°, such as 150° < a < 175°. Hence, in embodiments, monolithic converters in the monolayer may be arranged at an angle a, wherein 45° < a < 180°, such as 90° < a < 180°, especially 100° < a < 180°, more especially wherein 135° < a < 180°, such as 150° < a < 175°. The angle a is especially the (mutual) angle between the virtual planes facing away from the monolithic converters. In general, in embodiments of the invention the angle a may be the angle between the top surfaces of the adjacently configured monolithic converters (facing away from the monolithic converters). However, in embodiments wherein the top surfaces are not parallel to the virtual planes, the angle a may (slightly) differ from the angle between the top surfaces. In specific embodiments, a sensor element may be arranged between monolithic converters arranged at an angle a, especially wherein the angle a < 180°, more especially wherein part of the side surfaces of the adjacently configured monolithic converters touch and wherein the sensor element is configured at a different part of at least one of the touching side surfaces. Alternatively or additionally, the monolithic converters may be arranged at an angle such that the active collimator system may be arranged partially surrounding a part of a subject, such as a body part, especially a neck, an arm, or a leg. Hence, in embodiments, the monolithic converters of the set of two or more adjacently configured monolithic converters may have a height H and have virtual planes P perpendicular to the height H, wherein the virtual planes P of the set of two or more adjacently configured monolithic converters are arranged at an angle a, wherein 45° <a< 180°.

Adjacent monolithic converters within the monolayer may touch each other. Alternatively or additionally, between (other) adjacent monolithic converters there may be metal inserts. Alternatively or additionally, (other) adjacent monolithic converters within the layer may not touch each other, e.g. for providing (part of) a shared monolayer aperture. Especially, part of a (side) surface of a monolithic converter may touch part of a (side) surface of an adjacent monolithic converter, especially wherein the non-touching parts define a shared monolayer aperture. In embodiments, two adjacent monolithic converters may define an interface (such as how two cells in a grid define a gridline), wherein the two adjacent monolithic converters touch along parts of the interface and do not touch along one or more other parts of the interface thereby defining one or more shared monolayer apertures. In specific embodiments, each set of two adjacently configured monolithic converters defines at least one shared monolayer aperture of the coded aperture, i.e., a shared monolayer aperture is arranged in between each set of two adjacent monolithic converters. In alternative embodiments, not all sets of two adjacent monolithic converters define a shared monolayer aperture of the coded aperture.

In embodiments, the active collimator system may comprise a grid of adjacently configured monolithic converters, wherein the grid has grid cells and grid lines, wherein each of the grid cells comprises a single monolithic converter, wherein each monolithic converter has substantially identical dimensions to the grid cell, and wherein a plurality of monolayer apertures are arranged along grid lines of the grid of adjacently configured monolithic converters, and wherein each of the monolayer apertures is defined by a set of two or more adjacently configured monolithic converters. In further embodiments, each monolithic converter may fill about 85-100% of its corresponding grid cell, such as about 90-100%, like up to about 100%, such as especially 95-99%. In yet further or alternative embodiments, each grid cell may have approximately the same dimensions, especially each grid cell has identical dimensions.

In embodiments, the monolithic converters of a set of two or more adjacently configured monolithic converters may have an average volume V_m, wherein the set of two or more adjacently configured monolithic converters define a shared monolayer aperture having an aperture volume V_a, wherein V_a < V_m, especially V_a < 0.5*V_m, such as V_a < 0.1 *V_m, especially V_a < 0.05*V_m, such as V_a < 0.01 *V_m. In further embodiments, for each monolayer aperture the volume V_a < V_m, especially V_a < 0.5*V_m, such as V_a < 0. l *V_m, especially V_a < 0.05*V_m, such as V_a < 0.0l *V_m.

The monolithic converters are selected from the group consisting of scintillators and semiconductors, wherein the monolithic converters are especially configured to convert incident gamma rays and to provide a conversion product, wherein the sensor element is configured to detect the conversion product. Hence, in embodiments, at least some, especially all, of the monolithic converters may be scintillators, wherein the scintillators are configured to convert incident gamma rays and to provide scintillation photons, wherein the sensor element comprises a photosensor unit configured to detect the scintillation photons. In alternative or further embodiments, at least some, especially all, of the monolithic converters may be semiconductors, wherein the semiconductors are configured to convert incident gamma rays and to provide an electric current, wherein the sensor element comprises an electrode geometry configured to detect the electric current.

In specific embodiments, at least some, especially all, of the monolithic converters may be semiconductor scintillators, wherein the semiconductor scintillators are configured to convert incident gamma rays and to provide one or more of scintillation photons and an electric current, wherein the sensor element comprises a photosensor unit configured to detect the scintillation photons, and wherein the sensor element further comprises an electrode geometry configured to detect the electric current. During operation, the relative conversion of gamma rays to scintillation photons with respect to electric current may depend on an interaction location in the semiconductor scintillator. For example, if a gamma ray interacts near a top surface of the semiconductor scintillator the semiconductor scintillator may (primarily) provide an electric current, while if the gamma ray interacts further from the top surface of the semiconductor scintillator the semiconductor scintillator may provide scintillation photons. Typically, in such embodiments, the electric current and the scintillation photons may in embodiments be detected at the same time. The electric current and the scintillation photons may provide a different accuracy in the resolving of the position, energy and time of a gamma ray interaction. Hence, a benefit of a semiconductor scintillator may be that the position, energy and time may be beneficially resolved based on different sensor signals, for example, the position may be determined based on the electric current, whereas the energy and the time may be determined based on the scintillation photons. Hence, the idea is to use the best value from either the electric current or the scintillation photons for all three quantities.

In embodiments wherein at least some of the monolithic converters are scintillators, the scintillators may especially comprise scintillator crystals. Alternatively or additionally, the scintillators may especially comprise scintillator ceramics. Hence, the scintillators may comprise scintillator crystals or ceramics. Combinations of a plurality of different scintillators, with e.g. scintillators comprising scintillator crystals and other scintillators comprising scintillator ceramics, may also be applied. The scintillator may comprise one or more scintillating materials, especially a single scintillation material. Especially, the scintillator may comprise one or more of thallium activated sodium iodide (NATTI), bismuth germinate (BGO), cesium activated yttrium aluminum garnet (YAG:Ce), cesium activated lutetium aluminum garnet (LuAG:Ce), cadmium zinc telluride (CZT), lanthanum bromide (LaBn), REiSiOsiCe, wherein RE comprises especially one or more of Y, La, Lu, Gd and/or other rare earth elements, especially at least one or more of Y and Lu, etc., more especially wherein RE SiOvCe comprises Lu_2-xY_xSi0₅:Ce (LYSO). Alternatively or additionally, the scintillator may comprise A₄M₃O₁₂ material, wherein A comprises Bi and wherein M comprises one or more of Si and Ge, wherein at least part of M comprises Si. For instance, the single crystalline or ceramic A₄M₃O₁₂ material may comprise A₄(Gei-_xSi_x)₃0i₂, wherein 0. l<x<l, especially wherein x is at least 0.9. In specific embodiments, the single crystalline or ceramic A4M3O12 material may comprise (Bii-_yRE_y)₄M₃0i₂, wherein y is selected from the range of 0-0.2, and wherein RE refers to one or more rare earth elements.

In specific embodiments, wherein the monolithic converters comprise scintillators and wherein the sensor element comprises a photosensor unit, the sensor element may further comprise a light guide (also“wave guide”). The light guide may be configured to channel scintillation photons from a scintillator towards one or more photosensors in the photosensor unit. In addition, the light guide may especially be configured to reduce position-dependent differences in light collection efficiency. The light guide may be positioned in between the monolithic converter and the photosensor unit. Each light guide may be functionally coupled with a monolithic converter, especially with a plurality of monolithic converters, such as with 1-5,000 monolithic converters, such as at least 2, like at least 4, such as at least 16, especially at least 64. Hence, in embodiments, the light guide may be functionally coupled with 1-64 monolithic converters, such as 1-16, especially 1-4, such as 1-2. In specific embodiments, the light guide may be functionally coupled to 4 monolithic converters. The light guide may be functionally coupled with a photosensor unit. In this way, photons from the monolithic converters may propagate to the photosensor unit. The light guide may in embodiments have the shape of a plate and may be optically coupled, such as physically coupled to a side surface of the monolithic converter. Hence, the (length and width) dimensions of the light guide(s) may essentially be the same as that of the (respective) monolithic converter side surfaces, but may also essentially be the same as that of the combined dimensions of multiple side surfaces of adjacent monolithic converters.

The photosensor unit may be configured to receive and count scintillation photons emitted by a monolithic converter, wherein the monolithic converter is a scintillator. The photosensor unit may comprise a photosensor array. The photosensor unit may comprise one or more photosensors, especially a plurality of photosensors, such as 1-5,000,000,000 photosensors, such as at least 1, especially at least 2, such as at least 4, like at least 16, such as at least 64. The photosensors may comprise photomultiplier tubes. The photosensors may comprise digital photon counters. Also, the photosensors may comprise photodetectors, or hybrid photodetectors, or silicon-based Geiger-mode photodetectors. Especially, the photosensors may comprise silicon-based Photomultipliers (SiPMs), more especially the photosensors may comprise digital SiPMs. For instance, Philips Digital Photon Counting DPC3200 sensors may be applied. Hence, in embodiments the photosensor units comprise silicon photomultipliers (SiPM). SiPMs are a class of silicon single photon sensors based on single-photon avalanche diodes (SPAD). Especially, the SiPMs could be digital silicon photomultipliers (dSiPM) and/or digital photon counters (DPC). As known in the art, dSiPM and DPC, etc., may actually refer to the same class of devices, viz. SiPMs with integrated digital data acquisition, processing, and readout circuits.

In embodiments wherein at least some of the monolithic converters are semiconductors, the sensor element may comprise an electrode geometry. The electrode geometry may especially be configured to measure an electric current (provided by the semiconductors). During operation, the semiconductor may be interact with an incident gamma ray, especially convert the incident gamma ray to an electric current, wherein the electrode geometry may be configured to detect the electric current, especially wherein the electrode geometry detects one or more of a time, location and energy (transfer) of the interaction between the semiconductor and the gamma ray.

In embodiments, the electrode geometry may comprise one or more electrode geometries selected from the group comprising a single sided strip, a double sided strip, a pixelated array, or a hybrid pixel, or any other electrode geometry suitable for measuring an electric current provided by a semiconductor.

In yet further or alternative embodiments, any one of the monolithic converters may comprise an interior monolayer aperture of the coded aperture, i.e., a monolithic converter comprises a monolayer aperture that runs through the monolithic converter and does not provide part of the external edge. For instance, such interior monolayer aperture is not bordered by an adjacent monolithic converter, when comprised by a set of two or more adjacently configured monolithic converters (that may in embodiments define the shared monolayer aperture of the coded aperture). The interior aperture has an (interior) aperture edge, which is defined by a single monolithic converter (of the two or more adjacently configured monolithic converters). In embodiments, a monolithic converter may comprise a plurality of interior monolayer apertures. Hence, in yet a further aspect the invention also provides an active collimator system having a coded aperture, the active collimator system comprising (i) a monolithic converter, especially a monolayer of adjacently configured monolithic converters, wherein the monolithic converter(s) is (are) selected from the group consisting of scintillators and semiconductors, wherein the monolithic converter(s) is (are) configured to convert incident gamma rays and to provide a conversion product, and (ii) a sensor element configured to detect the conversion product; wherein the sensor element is configured at a surface of the monolithic converter or one or more of the monolithic converters, respectively, and wherein the monolithic converter( s) comprise an interior aperture (“through hole”) of the coded aperture. In specific embodiments, the interior aperture has an optical aperture axis O_a, wherein A_ai is a smallest cross-sectional area of the interior monolayer aperture perpendicular to the optical aperture axis O_a, and wherein A_mi is the largest cross- sectional area of the corresponding monolithic converter in a plane perpendicular to the optical aperture axis O_a. Especially, A_ai < 1.00* . _m, like A_H £ 0.5*A_mi, such as Ai £ 0.25* Ami, especially Ai £ 0.2*A_mi, such as Aai < 0. 1 * A_M even more especially A_ai < 0.05*A_mi, such as Aai < 0.0l *A_mi.

The features described herein with respect to one or more embodiments may further be beneficially combined. For example, in embodiments, the active collimator system may have a coded collimator aperture. The active collimator system may comprise a monolayer of adjacently configured monolithic converters may comprise two or more of: (i) bar-shaped monolithic converters, especially wherein a set of two or more adjacently configured bar shaped monolithic converters define a shared monolayer aperture having an optical axis O_a, wherein the shared monolayer aperture has a smallest cross- sectional area A_¾ (in a plane perpendicular to the optical axis O_a), and wherein the set of adjacently configured monolithic converters has a largest cross-sectional area A_m (in a plane perpendicular to the optical axis O_a), wherein 0.1 l*A_m< A_¾ < 1.5* A_m, such as 0. l l*A_m < A_¾ < 1.0* A_m; (ii) a metal collimator comprising a second coded aperture, wherein the second coded aperture comprises a metal collimator aperture, wherein the metal collimator is arranged such that the metal collimator aperture is aligned with the monolayer aperture of the set of two or more adjacently configured monolithic converters, and (iii) a sensor element configured in between adjacently configured monolithic converters of a set of two or more adjacently configured monolithic converters. It will be clear to a person skilled in the art that many more combinations of features described herein may be made within the scope of the invention.

In a second aspect, the current invention also provides a nuclear imaging system, the nuclear imaging system comprising the active collimator system as defined herein, the nuclear imaging system further comprising a gamma ray detector configured downstream of the active collimator system, wherein the gamma ray detector is configured to detect gamma rays that have passed through the coded aperture of the active collimator system. Hence, the gamma ray detector, especially its detector surface, may intercept an optical axis of the active collimator system. Especially, the nuclear imaging comprises a plurality of active collimator systems and (associated) gamma ray detectors. The term“downstream” refers to a relative position with respect to a gamma ray source during operation of the nuclear imaging system. The gamma ray source may especially be arranged at a staging area (for a subject), wherein the nuclear imaging system may be configured to detect gamma rays originating from the staging area. Hence, the gamma ray detector may be arranged further from a staging area (for a subject) than the active collimator system. In further embodiments, the nuclear imaging system may comprise the staging area (for a subject).

Nuclear imaging systems are especially specialized imaging systems for nuclear medicine. These systems are configured to locate radionuclide markers within a subject by detecting the gamma rays that these markers emit. The subject and the nuclear imaging system may move with respect to one another. The nuclear imaging system may comprise a moving gantry, especially wherein the moving gantry moves around the subject. The nuclear imaging system may comprise a moving subject area. The nuclear imaging system may comprise one or more gamma ray detectors. These gamma ray detectors may be assembled in a partial or whole ring around the subject. Instead of the term“subject area” also the terms“staging area” or“sensing area” may be applied.

A gamma ray detector is a device that detects gamma rays. The gamma ray detector may comprise a position-resolving gamma ray detector, i.e., a gamma ray detector that is configured to determine the origin location of an incident gamma ray. The gamma ray detector may comprise a position-and- energy-resolving gamma ray detector, i.e., a gamma ray detector that is configured to determine the origin location and the energy of an incident gamma ray. The gamma ray detector may comprise one or more scintillators and/or semiconductors.

The gamma ray detectors can e.g. measure one or more of the location, the time and the deposited energy of any gamma ray interaction.

The gamma ray detector may include essentially the same type of (monolithic) converters and sensor elements as the active collimator system; however, typically configured without aperture. Alternatively, the gamma ray detector may comprise other types of converters and/or sensor elements. In embodiments, the gamma ray detector may, for example, comprise a pixelated scintillator array and/or a pixelated semiconductor array.

Gamma ray detectors are known in the art. The gamma ray detector may especially comprise a gamma camera. Gamma cameras are known in the art. The gamma camera comprises a large scintillation crystal, especially a Nal(Tl) scintillation crystal, a light guide, and an array of photomultiplier tubes, and may further comprise a plurality of analog-to-digital converts and a collimator. The scintillation crystal is configured to absorb gamma rays and scintillate, thereby releasing scintillation photons. The light guide is functionally coupled to the scintillation crystal and is configured to distribute the scintillation photons to one or more of the photomultiplier tubes in the array. The photomultiplier tubes are configured to detect scintillation photons and provide an output signal, especially an analog output signal. The analog-to-digital converters are functionally coupled to the photomultiplier tubes and convert the analog output signal to an electronic output signal, especially to a digital output signal. The electronic output signal is further processes by a device, especially a computer, functionally coupled to the gamma camera. The collimator may be configured upstream of the scintillation crystal, and is configured to selectively absorb incoming gamma rays based on their incidence angle and location. The gamma camera is configured to position- and-energy-resolve incoming gamma rays.

The downstream gamma ray detector may be configured to detect gamma rays that have passed the active collimator system.

The nuclear imaging system may especially include a plurality of active collimator systems, each functionally coupled to one or more gamma ray detectors. The combination of collimator system and gamma ray detector may form a unit that may be configured at least partly rotatable around a sensing stage. The sensing stage may e.g. be configured to host a human.

Further, the nuclear imaging system may comprise or may be functionally coupled to a control system configured to control the nuclear imaging system. Further, the control system may be configured to control the combination) s) of active collimator system and gamma ray detector) s). Yet further, the control system may be configured to analyze the data generated by the gamma ray detector(s) and by the active collimator system.

Especially, the nuclear imaging system may comprise one or more of the anatomical imaging systems and the functional imaging systems, such as imaging systems for positron emission tomography (PET), single photon emission computed tomography (SPECT), computed tomography (CT), magnetic resonance imaging (MRI), ultrasound, tomosynthesis, optical fluorescence, magnetic particle imaging (MPI), electroencephalography (EEG), Electrocardiography (ECG) etc.. More especially, the nuclear imaging system may comprise or be functionally coupled to one or more of a positron emission tomography (PET) imaging system, a single photon emission computed tomography (SPECT) imaging system, a computed tomography (CT) imaging system, and a magnetic resonance imaging (MRI) imaging system. Hence, in embodiments the nuclear imaging system comprises a PET-SPECT detector system with active collimation. This may e.g. allow measuring gamma-rays emitted from multiple SPECT and PET molecular vectors simultaneously. In further embodiments, the nuclear imaging system comprises a PET-SPECT detector system, and may further comprise or be functionally coupled to one or more other anatomical and/or functional imaging systems. In embodiments, the nuclear imaging system comprises a PET-SPECT-CT imaging system or PET- SPECT-MRI imaging system capable of measuring multiple molecular vectors simultaneously, whilst integrated into an X-ray CT or MRI gantry to obtain anatomical information, and yield an entirely new dimension of physiological information for clinicians to use in diagnosis and treatment. An example of this would be the measurement of multiple biological processes simultaneously, such as metabolism, hypoxia, apoptosis, DNA damage repair, etc., to determine regions of radio resistance within tumors during radiotherapy treatment. Hence, the imaging system may in embodiments further optionally comprise one or more other anatomical and/or functional imaging systems (or functionally be coupled to such systems).

In a further aspect the invention further provides a method for the determination of an emission location (“gamma ray source location”) of an incident gamma ray using an active collimator system or a nuclear imaging system or a monolithic converter according to the invention. The method may further comprise one or more image formation methods selected from the group comprising mechanical collimation, coincidence/electronic collimation, and Compton collimation, the method may especially comprise two or more, such as three, image formation methods selected form the group comprising mechanical collimation, coincidence/electronic collimation, and Compton collimation.

In embodiments, the method may be applied for one or more of treatment delivery monitoring (in radiotherapy treatment), radiotherapy dose delivery monitoring, prompt gamma ray analysis, PET imaging in proton therapy, nuclear medicine, homeland security detection systems, and customs detection systems. In specific embodiments, the method comprises monitoring treatment delivery in a radiotherapy treatment, especially treatment dose delivery in a radiotherapy treatment. In alternative specific embodiments, the method comprises the detection of radioactive material, especially the detection of radioactive material at customs or at a security checkpoint, such as at a homeland security checkpoint.

In embodiments, the method may be a non-medical method.

Radiotherapy treatment refers to the use of (ionizing) radiation as a treatment rather than for imaging (as in nuclear imaging). Radiation therapy may be used as (part of) a treatment for, for example, cancer, Dupuytren’s disease, Ledderhose disease, or for part of a procedure for a bone marrow transplantation. The active collimator system and the nuclear imaging system as described herein may be used to monitor the treatment delivery in a subject, i.e., assess whether a sufficient fraction of the radiation sources arrive at the target tissue and to assess which other tissues may be exposed to the radiation. The active collimator system and the nuclear imaging system according to the invention may, for example, be applied in one or more of proton radiotherapy, photon radiotherapy, or any other form of external or internal radiotherapy treatment.

In yet a farther aspect, the invention further provides a monolithic converter as described herein in various embodiments. Especially, in embodiments the monolithic converter is selected from the group consisting of scintillators and semiconductors. Especially, in embodiments the monolithic converter has a first volume V 1, wherein a largest cuboid fitting within the monolithic converter has a second volume V2, wherein 0.8<V2/Vl<l, especially 0.9<V2/Vl<l, wherein the monolithic converter has one or more of (i) a rectangular planar surface including a recess and (ii) a truncated edge.

Especially, the monolithic converter may be configured for use in the active collimator system as defined herein. The rectangular planar surface may especially be a rectangular planar side surface, especially wherein the recess runs along a height dimension of the side surface. The recess may be configured such that a shared monolayer aperture may be formed when the respective side surface is placed against an adjacent monolithic converter. Hence the recess may especially define a cavity.

The term“tamcated edge” refers to an edge and/or surface resulting from a truncation, i.e., resulting from an operation in any dimension that cuts polytope edges, creating a new surface bordering each of the cut polytope edges. The tamcated edge especially comprises a new surface that is not parallel to a top or side surface of the monolithic converter, i.e., the new surface is especially a slanted surface.

In a fifth aspect the invention further provides a monolayer of adjacently configured monolithic converters, wherein the monolithic converters are selected from the group consisting of scintillators and semiconductors, wherein the monolithic converters are configured to convert incident gamma rays and to provide a conversion product, and wherein a set of two or more adjacently configured monolithic converters define a shared monolayer aperture of the coded aperture, wherein the shared monolayer aperture has an optical aperture axis O_a, wherein A_a is a smallest cross-sectional area of the monolayer aperture perpendicular to the optical aperture axis O_a, and wherein A_m is the largest cross- sectional area of the set of two or more adjacently configured monolithic converters in a plane perpendicular to the optical aperture axis O_a, and wherein A_a < l .00*A_m, like A_a < 0.5*A_m, such as A_a < 0.25*A_m, especially A_a < 0.2*A_m, such as Aa < 0.1 * Am even more especially A_a < 0.05*A_m, such as Aa < 0.0l *A_m..

In a sixth aspect the invention further provides a method for manufacturing a monolayer of monolithic converters according to the invention, wherein the method comprises the steps: (i) a monolithic converter production step comprising producing a plurality of monolithic converters, wherein the monolithic converters are selected from the group consisting of scintillators and semiconductors; (ii) removing one or more sections of one or more of the plurality of monolithic converters; (iii) arranging the monolithic converters side-by-side such that a shared monolayer aperture is formed in between a set of two or more adjacently configured monolithic converters. In embodiments, the method may further comprise (iv) arranging a sensor element on a surface of one or more of the adjacently configured monolithic converters. In further embodiments, step (ii) may comprise removing one or more sections of one or more of the plurality of monolithic converters, wherein one or more cutting methods selected from the group comprising laser cutting, water jet cutting, and diamond edge cutting is used to remove the one or more sections. Wherein the monolithic converter production step may comprise any method for the production of scintillators or semiconductors suitable for gamma ray conversion known in the art including, for example, growing scintillator crystals. In specific embodiments, the method comprises growing scintillator crystals against a metal surface, especially wherein the metal surface is the surface of a metal collimator. Hence, in specific embodiments, the scintillator crystals may be grown on top of a metal collimator.

The herein described embodiments related to one aspect of the invention may further apply to one or more other aspects of the invention. Hence, the embodiments described herein are not limited to a single aspect of the invention. For example, an embodiment describing the monolayer of monolithic converters of the active collimator system may further apply to the fifth aspect of the invention. Similarly, an embodiment describing details of a monolithic converter may further apply to the fourth aspect of the invention. It will be clear to a person skilled in the art how the embodiments described herein relate to the different aspects of the invention.

The terms“upstream” and“downstream” relate to an arrangement of items or features relative to the propagation of radiation from a radiation generating means (here especially the gamma ray source), wherein relative to a first position within a ray of radiation from the gamma ray generating means, a second position in the ray of radiation closer to the gamma ray generating means is“upstream”, and a third position within the ray of radiation further away from the gamma ray generating means is“downstream”.

The active collimator system may be part of or may be applied in e.g. gamma ray detectors, gamma ray cameras, nuclear imaging systems, PET systems, SPECT systems, PET-CT systems, PET-MRI systems, SPECT-CT systems, SPECT-MRI systems, homeland security detection systems, radiotherapy dose delivery monitoring systems, prompt gamma imaging, and activated PET imaging.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

Fig. 1A schematically depicts a cross-section of a parallel-hole collimator; Fig. 1B schematically depicts a cross-section of a pinhole collimator; Fig. 1C schematically depicts cross-sections of both a converging collimator and a diverging collimator;

Fig. 2A-B schematically depict a cross-sectional side view (2A) and a top view (2B) of an embodiment of the active collimator system.

Fig. 3 A schematically depicts a top view of an alternative embodiment of the active collimator system. Fig. 3B schematically depicts a cross- sectional side view of a monolithic converter of the active collimator system depicted in 3A.

Fig. 4A schematically depicts a top view of an alternative embodiment of the active collimator system. Fig. 4B schematically depicts a cross- sectional side view of a set of two or more monolithic converters of the active collimator system depicted in 4A.

Fig. 5 schematically depicts a cross-sectional side view of an embodiment of the active collimator system further comprising a metal collimator.

Fig. 6 schematically depicts two sets of adjacently configured monolithic converters, wherein the adjacently configured monolithic converters of the different sets are arranged at different angles.

Fig. 7A-C schematically depict embodiments of the nuclear imaging system comprising the active collimator system.

Fig. 8A-B schematically depict cross-sectional areas of shared monolayer apertures and the corresponding cross-sectional areas of the adjacently configured monolithic converter.

The schematic drawings are not necessarily on scale. DETAILED DESCRIPTION OF THE EMBODIMENTS Figs 1A-1C schematically depict cross-sections (along an optical axis O) of a non-limiting number of embodiments of shapes of collimator apertures 201 of collimators 50.

Fig. 1A schematically depicts a cross-section of a collimator 50 having a collimator aperture 201 with a parallel-hole shape 116. The collimator has a collimator aperture 201 and comprises collimator material 200. The collimator 50 further has two outer planes 203, 204 at opposite ends of the collimator material 200 and perpendicular to an optical axis O of the collimator, wherein the outer planes 203, 204 are separated by a distance Ll parallel to the optical axis O. The optical axis O of the collimator is especially parallel to the optical aperture axis O_a (not drawn) of each of the apertures. The collimator aperture 201 has a parallel-hole shape 1 16 and comprises a plurality of parallel apertures (or openings). Especially, the parallel apertures may be parallel to the optical axis O. These parallel apertures are separated by collimator material 200.

Fig.1B schematically depicts a cross-section of a collimator 50 having a collimator aperture 201 with a pinhole shape 1 17. The collimator 50 has a collimator aperture 201 and comprises collimator material 200. The collimator 50 further has two outer planes 203, 204 at opposite ends of the collimator material

200 and perpendicular to an optical axis O, wherein the outer planes 203, 204 are separated by a distance Ll parallel to the optical axis O. The collimator aperture

201 has a pinhole shape 117. Especially, the collimator aperture 201 is shaped as an hourglass along the optical axis O. In this embodiment, the optical aperture axis O_a (not drawn) coincides with the optical axis O. In other embodiments, the collimator aperture 201 may also be shaped as a partial hourglass. Especially, the collimator aperture 201 may be cone-shaped, such as the hourglass shape as depicted. More especially, each cross-section of the collimator aperture 201 perpendicular to the optical axis O is a circle.

Fig. 1C schematically depicts a cross-section of an embodiment of a collimator 50 having a collimator aperture 201 with either a converging shape 118 or a diverging shape 1 19 (dependent upon the view direction along an optical axis O). Hence, the converging shape 118 and the diverging shape 119 may be mirror images of each other. The collimator 50 has a collimator aperture 201 and comprises collimator material 200. The collimator 50 further has two outer planes 203, 204 at opposite ends of the collimator material 200 and perpendicular to the optical axis O, wherein the outer planes 203, 204 are separated by a distance L l parallel to the optical axis O. The collimator aperture 201 comprises a plurality of non-parallel apertures having a downstream aperture end and an upstream aperture end. The upstream aperture ends and the downstream aperture ends of the non parallel apertures coincide with the outer planes 203, 204, wherein‘upstream’ and ‘downstream’ indicate a relative position to a gamma ray emitting source along the optical axis outside of the collimator 50. The non-parallel apertures are configured having an aperture angle b with the optical axis O. Hence, in this embodiment, the optical axis O is not parallel to each of the optical aperture axes O_a (not drawn) of the apertures. In embodiments, the optical axis O may especially be a weighted average of the optical aperture axes O_a. Especially, the non-parallel apertures are configured such that the aperture angle b increases with increasing distance to the optical axis O. More especially, two adjacent non-parallel apertures have angles b, bi, b₂, wherein the aperture closer to the optical axis O has angle bi and the aperture further from the optical axis has angle b₂ and bi < b₂. The collimator aperture 201 having a converging shape 118 is configured such that the distance of the upstream aperture end to the optical axis O is smaller than the distance of the downstream aperture end to the optical axis O. The collimator aperture 201 having a diverging shape 1 19 is configured such that the distance of the upstream aperture end to the optical axis O is larger than the distance of the downstream aperture end to the optical axis O.

Embodiments also include collimators 50 having collimator apertures 201 shaped according to two or more of the parallel-hole shape 1 16, the pinhole shape 117, the converging shape 118, and the diverging shape 119. Examples include collimators 50 having a collimator aperture shaped according to one or more of a fan beam collimator, a multi-pinhole collimator, or a slit-slat collimator.

Fig. 2A schematically depicts a cross-sectional side-view of an embodiment of the active collimator system 100 having a coded aperture 110, the active collimator system 100 comprising (i) a monolayer 105 of adjacently configured monolithic converters 120, wherein the monolithic converters 120 are selected from the group consisting of scintillators and semiconductors, wherein the monolithic converters 120 are configured to convert incident gamma rays 160 and to provide a conversion product 161, and (ii) a sensor element 130 configured to detect the conversion product 161; wherein the sensor element 130 is configured at a surface of one or more of the monolithic converters 120, and wherein a set 125 of two or more adjacently configured monolithic converters defines a shared monolayer aperture 140,141 of the coded aperture 110, wherein the shared monolayer aperture 140, 141 has an optical aperture axis O_a, wherein A_a is a smallest cross-sectional area of the monolayer aperture 140,141 perpendicular to the optical aperture axis O_a, and wherein A_m is the largest cross-sectional area of the set 125 of two or more adjacently configured monolithic converters with a plane perpendicular to the optical aperture axis O_a, and wherein A_a < l .00*A_m, like A_a < 0.5* A_m, such as A_a < 0.25 *A_m, especially A_a < 0.2*A_m, such as A_a < 0. l *A_m even more especially < 0.05*A_m, such as A_a < 0.0l *A_m. In the depicted embodiment, the monolithic converters 120 are scintillators and the conversion product 161 comprises scintillation photons.

In the embodiment, the slanted surfaces 123 of the monolithic converters 120 define shared monolayer apertures 140, 141 having a pinhole shape. Hence, at least one of the monolithic converters 120 of a set 125 of two or more adjacently configured monolithic converters comprises a slanted surface 123, wherein the slanted surface 123 defines at least part of the shared monolayer aperture 140, 141. In the depicted embodiment, the shared monolayer apertures 140 are regularly arranged with parallel optical axes. Hence, the monolayer 105 has a coded aperture 1 10 with both a parallel-hole shape 116 and a pinhole shape 117. In alternative or further embodiments, the coded aperture may have one or more of a parallel-hole shape, a pinhole shape, a converging shape, or a diverging shape.

The monolithic converters have a height H, and have a first length Lmi perpendicular to the height H. The monolithic converters 120 of the set 125 of two or more adjacently configured monolithic converters have a height H and have virtual planes P perpendicular to the height H, wherein the virtual planes P of the set of two or more adjacently configured monolithic converters are arranged at an angle a, wherein 45° <a< 180°. In the depicted embodiment, a =180°. In the depicted embodiment, the optical aperture axis O_a of the monolayer apertures 140 is parallel to the height H of the monolithic converters 120. Hence, in the depicted embodiments the virtual planes P are parallel to the outer planes 203,204. In alternative embodiments, the optical aperture axis O_a may not be parallel to the height H, and the virtual planes P may not be parallel to the outer planes 203,204.

In the depicted embodiment, the sensor element 130 is configured in between adjacently configured monolithic converters 120 of the set 125 of two or more adjacently configured monolithic converters.

Fig. 2B schematically depicts a top view of a section of an embodiment of the active collimator system 100. The active collimator system 100 of Fig. 2 A may comprise a plurality of the active collimator system 100 depicted in Fig. 2B. The top view of the active collimator system 100 schematically displays the view of the active collimator system 100 from a gamma ray source. The view may be dominated by the top surfaces 121 of the monolithic converters 120 and may further comprise slanted surfaces 123 of the monolithic converters 120 defining cavities 124. In the depicted embodiment, a shared monolayer aperture 140, 141 is formed at the intersection of four monolithic converters 120. The shared monolayer aperture 140, 141 is defined by the slanted surfaces 123 and the cavities 124 of the monolithic converters 120 of the set 125 of adjacently configured monolithic converters. Hence, in the embodiment a set 125 of three or more adjacently configured monolithic converters defines at least one shared monolayer aperture 140,141 of the coded aperture 1 10, wherein the at least one shared monolayer aperture of the coded aperture is formed by adjacently configured cavities in all of the three or more adjacently configured monolithic converters of the set. In the depicted embodiment, two of the monolithic converters 120 comprise an interior monolayer aperture 140, 142 of the coded aperture. Hence, a single monolithic converter 120 may both comprise one or more interior monolayer apertures 140, 142 and may define one or more shared monolayer apertures 140, 141 as part of a set 125 of two or more monolithic converters. Fig 2A and Fig 2B outline an exemplar active collimator system 100 comprising a monolayer 105 of adjacently configured monolithic converters 120 for medium resolution/sensitivity SPECT-PET imaging. Here a number of pinhole like structures are created through the slanted nature of the monolithic converter surfaces. These monolayer aperture 140 shapes limit the solid angle with which emitted gamma rays 160 from a SPECT molecular tracer can pass through replicating the image formation process key to SPECT imaging. Whereas since the collimator 50 is active, any 51 1 keV gamma-ray from the PET tracer which interacts with the collimator 50, especially with a monolithic converter 120, may result in a recording of one or more of its energy deposition, interaction location and time. This data may then be leveraged with the measurements in other monolithic converters 120, or - when applied, for example in a nuclear imaging system - with a coupled planar position-and-energy-resolving radiation detector, to reconstruct these events to pair with their partner co-linear 51 1 keV gamma-ray to perform PET reconstruction. The result is a SPECT-PET imaging system with improved PET sensitivity whilst achieving similar SPECT resolution to metal collimator designs which may currently be implemented in clinics.

Fig. 3A schematically depicts a top view of an alternative embodiment of the active collimator system, wherein side surfaces 122 of the monolithic converters 120 resemble grid lines, and wherein monolayer apertures 140 are arranged along the grid lines. In the depicted embodiment, a (first) set l25a of two adjacently configured monolithic converters defines at least one shared monolayer aperture l40,l4 la of the coded aperture 1 10. Especially, the at least one shared monolayer aperture 140, 14 la of the coded aperture 110 is formed by adjacently configured cavities 124 in both adjacently configured monolithic converters 120 of the set 125 of two or more adjacently configured monolithic converters. In this embodiment, the active collimator system 100 comprises a plurality of sets l25, l25a of two or more adjacently configured monolithic converters, wherein each set l25,l25a of two or more adjacently configured monolithic converters defines at least one shared monolayer aperture 140, 14 la of the coded aperture 1 10. In further embodiments, each two adjacently configured monolithic converters 120 may be comprised in a set !25,l25a of two or more adjacently configured monolithic converters, wherein each set l25, l25a of two or more adjacently configured monolithic converters defines at least one shared monolayer aperture l40, l4la of the coded aperture 1 10.

In alternative or further embodiments at least one shared monolayer aperture 140, 141 of the coded aperture 110 may be formed by a single cavity 124 in one of the adjacently configured monolithic converters 120 of the set 125 of adjacently configured monolithic converters. The depicted embodiment further comprises monolayer apertures 140 formed at the intersections of three or more adjacently configured monolithic converters 120, especially monolayer apertures 140 defined by a (second) set l25b of three or more, especially four, adjacently configured monolithic elements. Hence, the depicted active collimator system 100 comprises one or more sets l25a of two adjacently configured monolithic elements and comprises one or more sets l25b of three or more adjacently configured monolithic elements. In the depicted embodiment, not all adjacently configured monolithic converters define a shared monolayer aperture defined by two adjacently configured monolithic converters.

Fig. 3B schematically depicts a cross-section of one of the monolithic converters of Fig. 3A, especially a cross-section at or close to a side surface 122 of the monolithic converter 120. In the depicted embodiment, a sensor element 130 covers the bottom surface of the monolithic converter 120. In alternative embodiments, the sensor element 130 may be configured at a part of the surface of one or more of the monolithic converters 120. In this embodiment, the sensor element 130 may be functionally coupled to a single monolithic converter 120. In general, each monolithic converter 120 may be functionally coupled to a sensor element 130. Hence, in such embodiment, the active collimator system 100 may comprise a plurality of sensor elements 130, wherein each of the sensor elements 130 is functionally coupled to one monolithic converter 120, and wherein each of the monolithic converters 120 is functionally coupled to one of the sensor elements 130.

In the embodiment depicted in Fig. 3A-B, each of the monolithic converters has a height H, wherein H is smaller than the longest length of the respective monolithic converter in each dimension perpendicular to H. From the depicted embodiments in Fig. 3A-B it will be clear to one skilled in the art that each of the monolithic converters 120 in the embodiment approximates a cuboid shape, wherein cavities along side surfaces 122 of the cuboid shapes define shared monolayer apertures 140,141 between adjacently configured monolithic converters 120. It will further be clear to one skilled in the art that the shared monolayer apertures 140, 141 may be substantially smaller than the monolithic converters 120. Especially, the smallest cross-sectional area A_a of the monolayer apertures may be substantially smaller than the largest cross-sectional area A_m of the set of adjacently configured monolithic converters, such as A £ 0.5*A_m, especially £ 0.2* Am such as At £ 0. l *A_m.

In alternative embodiments, the height H may be larger than the longest length of the respective monolithic converter in each dimension perpendicular to H. In further or alternative embodiments, each monolithic converter unit may have a size selected from the range of 2 x 2 x 2 mm - 48 x 48 x 48 mm.

In the embodiment of Fig. 3A-B, the monolithic converter 120 may e.g. have a first volume VI, wherein VI equals L_mi*L_m2*H, i.e., VI has a cuboid shape with height H, a first length L_ml perpendicular to the height H and a second length Lu_tz perpendicular to both H and to L_mi. Hence, VI comprises the monolithic converter 120 as well as its cavities 124. The monolithic converter further has a second volume V2, wherein V2 is the largest cuboid fitting in the monolithic converter 120, i.e., V2 does not comprise the cavities 124 of the monolithic converter 120. In embodiments, 0.8<V2/Vl<l, especially 0.9<V2/Vl<l . The monolithic converter has a rectangular planar surface, i.e., its top surface 121 approximates a rectangular planar surface but for the cavities 124, i.e., VI has a rectangular planar surface.

Fig. 4A depicts a top-view of an embodiment of the active collimator system, wherein along a dimension of the monolayer 105 of adjacently configured monolithic converters 120, the monolithic converters alternate in containing or not containing a cavity, thereby providing apertures defined by cavities in two monolithic converters, while also defined by surfaces of six adjacently configured monolithic converters. Fig. 4B depicts a side view of the embodiment of Fig. 4A, illustrating that the aperture in this embodiment has a constant cross-sectional area along its optical aperture axis O_a. In the embodiment of Fig. 4A-B, the sensor element 130 is functionally coupled to a plurality of monolithic converters 120.

In the embodiments depicted in Fig. 2A-B, Fig. 3 A-B, and Fig. 4A- B the active collimator system may exclusively comprise a non-magnetic material. Hence, the embodiments depicted therein may be constructed to be both MRI and X-ray CT compatible. Embodiments of the active collimator system comprising magnetic materials may be constructed to be X-ray CT compatible.

However, in the embodiment depicted in Fig. 5 the active collimator system 100 further comprises a metal collimator 180 comprising a second coded aperture 112, wherein the second coded aperture 1 12 comprises a metal collimator aperture 185, wherein the metal collimator 180 is arranged such that the metal collimator aperture 185 is aligned with the shared monolayer aperture 140, 141 of the set 125 of two or more adjacently configured monolithic converters, especially wherein a plurality, more especially all, of the metal collimator apertures 185 are aligned with respective shared monolayer apertures 140, 141 of a plurality of sets 125 of two or more adjacently configured monolithic converters. In further or alternative embodiments, the active collimator system 100 may further comprise a metal insert arranged between two or more adjacent monolithic converters 120 to further define a shared monolayer aperture 140, 141. The outer planes 203, 204 (not drawn) of the active collimator system are separated by a distance Ll, wherein Li is essentially the sum of the height H of the monolithic converters combined and the height of the metal collimator.

Fig. 5 outlines a possible system design that may be particularly suitable for whole body high resolution, medium sensitivity SPECT-PET imaging, and radiotherapy dose delivery monitoring (prompt gamma-ray and active PET imaging in proton therapy). Here a monolayer of monolithic converters are placed in front of an (encoded) metal collimator, and - when applied, for example in a nuclear imaging system - of a further downstream energy- spatially resolving radiation detector. The aperture shapes limit the solid angle with which emitted gamma rays from a SPECT molecular tracer/prompt gamma rays created from proton interaction can pass through replicating the image formation process key to SPECT and prompt gamma ray imaging. Furthermore, any 51 1 keV gamma ray from a PET source (activated and molecular tracer) that interacts within the active elements may result in a recording of its energy deposition, interaction location and time. This data may then be leveraged with measurements in other monolithic converters or in the downstream position-and-energy-resolving radiation detector to reconstruct these interaction events to pair with their partner co-linear 51 1 keV gamma-ray to perform PET reconstaiction. In these examples the (encoded) metal collimator acts as the primary image defining element for SPECT/prompt gamma ray sources and can be produced to match the performance of any other metal collimator. This approach of mixed collimation may help to increase the systems operating range up to, for example, ~l MeV for molecular imaging and ~20 MeV for radiotherapy dose delivery monitoring.

Fig. 6 schematically depicts an embodiment of the monolayer of adjacently configured monolithic converters wherein a first set 125a of two or more adjacently configured monolithic converters are arranged at an angle ou ~ 140°, and wherein a second set l25b of two or more adjacently configured monolithic converters are arranged at an angle a 2 ~ 180°. The monolithic converters 120 of the first set l25a of adjacently configured monolithic converters define a shared monolayer aperture 140,141 having a first optical aperture axis Oai . In this embodiment, for the first set l25a of adjacently configured monolithic converters, the smallest cross-sectional area An of the shared monolayer aperture 140, 141 perpendicular to Oi lies in a different plane than the largest cross-sectional area A_mi of the set 125 a of two or more adjacently configured monolithic converters perpendicular to Oi . The monolithic converters 120 of the second set l25b of adjacently configured monolithic converters define a shared monolayer aperture 140,141 having a second optical aperture axis O_a2. In this embodiment, for the second set l25b of adjacently configured monolithic converters, the smallest cross-sectional area An of the shared monolayer aperture 140, 141 perpendicular to O2 lies in the same plane as the largest cross-sectional area A_m2 of the set l25b of two or more adjacently configured monolithic converters perpendicular to 0₂. Hence, in embodiments, A and A_m may lie in the same plane (e.g. the middle and right converters 120). Additionally or alternatively, (different instances of) A_:i and A_m may lie in different planes (e.g. the left and middle converters 120). Hence, A_m may be a summation of the cross-sections of the individual adjacently configured monolithic converters (defining the aperture) with the (same) (virtual) plane.

In embodiments of the active collimator system 100 of any one of Fig. 2A-6, all of the monolithic converters 120 may be scintillators, wherein the scintillators are configured to convert incident gamma rays 160 and to provide scintillation photons, wherein the sensor element 135 comprises a photosensor unit configured to detect the scintillation photons.

In alternative embodiments of the active collimator system 100 of any one of Fig. 2A-6, all of the monolithic converters 120 may be semiconductors, wherein the semiconductors are configured to convert incident gamma rays 160 and to provide an electric current, wherein the sensor element 135 comprises an electrode geometry configured to detect the electric current.

Fig. 7 A depicts an embodiment of the nuclear imaging system 1000 during operation, the nuclear imaging system 1000 comprising the active collimator system 100 according to the invention, the nuclear imaging system 1000 further comprising a gamma ray detector 1040 configured downstream of the active collimator system 100 (with respect to a staging area and/or a gamma ray source), wherein the gamma ray detector 1040 is configured to detect gamma rays 160 that have passed through the coded aperture 1 10 of the active collimator system 100. The gamma ray source, especially a PET source 1072 or a SPECT source 1073, is arranged at a staging area 1070 (for a subject), wherein the nuclear imaging system 1000 is configured to detect gamma rays 160 originating from the staging area 1070. Hence, the gamma ray detector 1040 is arranged further from the staging area 1070 than the active collimator system 100.

During measurement with the nuclear imaging system 1000, a human or animal, or other subject, may be at the staging area 1070. The object under investigation may include a PET source 1072, especially positron emitting radionuclide markers, and/or a SPECT source 173, especially gamma ray emitting radionuclide markers. Hence, during measurement the staging area 170 may comprise a locally accumulated PET source 1072 and a locally accumulated SPECT source 1073. The PET source 1072 emits positrons. The positrons travel a small distance from the PET source 1072 before encountering an electron and undergoing a positron annihilation event, resulting in the emission of two paired gamma rays 160, 162 in roughly opposite directions. The SPECT source 1073 emits unpaired gamma rays 160, 163. Each active collimator system 100 is configured to either reject or accept incoming gamma rays 160, especially paired gamma rays 160, 162, or especially unpaired gamma rays 160, 163, based on their incidence angle and location. Rejected gamma rays 160 are absorbed, detected and measured by the active collimator system 100. Accepted gamma rays 160 pass through the active collimator system 100. Each of the gamma ray detectors 1040 is configured downstream of one or more active collimator systems 100 to detect and measure gamma rays 160 that pass through the one or more active collimator systems 100. It is clear to a person skilled in the art that some gamma rays may penetrate the active collimator system 100. The gamma ray detectors 1040 may be configured to detect and measure gamma rays 160 that penetrate the active collimator system 100. Especially, the gamma ray detectors 1040 may be configured to detect and measure paired gamma rays 160, 162 that penetrate the active collimator system. The rays herein are only indicated by way of example.

Fig. 7A illustrates the active collimation process of the nuclear imaging system 1000 comprising the active collimator system 100. Here a pair of the exemplar pinhole-like active collimator systems 100 have each been coupled to position-and-energy-resolving gamma ray detectors 1040 and are directed to face a test object with a 180° separation. In Fig. 7A the SPECT source 1073 and the PET source 1072 can be seen to emit single - 150 keV unpaired gamma rays 160, 163 and 51 1 keV paired gamma rays 160, 162, dashed— for the unpaired gamma rays 160, 163, and - for the paired gamma rays 160, 162 respectively, from their respective gamma ray source locations. These gamma rays 160 interact both within the active collimator system 100 and with the downstream radiation detectors 1040 such that their energy, interaction position and time can be measured. These interaction locations may then be filtered based on the detected energy and interaction time to construct a set of gamma ray detection event locations that could have come from either the SPECT source 1073 or from the PET source 1072. Filtering on the SPECT gamma ray detection event locations (those corresponding to unpaired gamma rays 160,163) and ignoring those within the active collimator system 100, as it may be difficult to form CoRs/LoRs from a single interaction in the active collimator system 100, it is possible to form LoRs with the edges of the pinhole-like openings. By backprojecting these LoRs into the system imaging space an estimate of the SPECT source location 1073 may be formed (not depicted). Whereas the LoRs that estimate the PET source 1072 location can be constructed and backprojected into image space via the application of time filtering of the remaining paired gamma rays 160, 162 detection event locations both in the active collimator system and the downstream gamma ray detectors.

Fig. 7B schematically depicts a cross-section of another embodiment of the nuclear imaging system 1000, wherein the active collimator system 100 and the gamma ray detector 1040 are integrated into a housing 1050. In embodiments, the housing 1050 may comprise one or more metals, especially one or more metals selected from the group comprising platinum, tungsten, lead, gold, iridium, rhenium, tantalum, rhodium, ruthenium and molybdenum, more especially platinum, tungsten, lead, molybdenum and gold, yet more especially the same metal as the metal collimator 180. In the depicted embodiment the active collimator system 100 comprises a monolayer 105 of adjacently configured monolithic converters 120, wherein the monolithic converters 120 are bar-shaped monolithic converters 120 having a height H larger than a length in any other dimension perpendicular to H, for example the height H is larger than the first length L_mi perpendicular to the height H. Further, especially a set of two or more adjacently configured (bar-shaped) monolithic converters define a shared monolayer aperture having an optical axis O_a, wherein the shared monolayer aperture has a smallest cross-sectional area A (in a plane perpendicular to the optical axis O_a), and wherein the set of adjacently configured monolithic converters has a largest cross-sectional area A_m (in a plane perpendicular to the optical axis O_a), wherein 0.3*A_m< A < l.5*A_m. The active collimator system 100 further comprises a metal collimator 180 comprising a second coded aperture 112, wherein the second coded aperture 1 12 comprises a metal collimator aperture, wherein the metal collimator is arranged such that the metal collimator aperture is aligned with the shared monolayer aperture 140, 141 of the set 125 of two or more adjacently configured (bar-shaped) monolithic converters. In this embodiment, the active collimator system may further comprise a sensor element 130 configured in between adjacently configured monolithic converters 120 of a set 125 of two or more adjacently configured monolithic converters, i.e., in embodiments the sensor element may be configured at side surfaces between the adjacently configured monolithic converters.

Fig. 7C schematically depicts another embodiment of the nuclear imaging system 1000 comprising the active collimator system 100. In this embodiment, the adjacently configured monolithic converters 120 of the monolayer of adjacently configured monolithic converters 120 are arranged at an angle a such that the active collimator system 100 may be arranged around the staging area 1070. In this embodiment, at least two of the adjacently configured monolithic converters 120 are placed at an angle on, i.e., the virtual planes Pn and P i2 perpendicular to their respective heights Hu and H12 are arranged at an angle ai, wherein ai -150°. In the same embodiment, other adjacently configured monolithic converters are arranged at a different angle. In this embodiment, at least two of the adjacently configured monolithic converters 120 are placed at an angle on i.e., the virtual planes P21 and P22 perpendicular to their respective heights H21 and H₂₂ are arranged at an angle 02, wherein 02 ~ 180°. In the embodiment, shared monolayer apertures !40, l40b may be defined by sets 125 of adjacently configured monolithic converters configured at an angle ai, and shared monolayer apertures l40,l40a may be defined by sets 125 of adjacently configured monolithic converters configured at an angle 0.2. In further embodiments, the sensor element 135 may be configured in between monolithic converters arranged at an angle on, especially at a side surface or at a slanted surface, more especially wherein the sensor element 135 is arranged at a part of the surface of one or more of the monolithic converters 120. In further or alternative embodiments, the sensor element 135 may be arranged at a top and/or at a bottom surface 121 of a monolithic converter 120. Fig. 8A schematically depicts a top view of a cross-section of an embodiment comprising shared monolayer apertures 140, 141 c, 141 d, and corresponding sets l25c, l25d of two or more adjacently configured monolithic converters. In this embodiment, the monolithic converters 120 have a cuboid shape; especially the cross-section of the monolithic converters along their height approximates a square. Hence, the monolayer of adjacently configured monolithic converters 120 resembles a square grid having cells, gridlines, and intersections. A third set l25c of two or more, especially four, adjacently configured monolithic converters defines a third shared monolayer aperture 140, 141 c at their intersection via cavities arranged at their shared intersection. The third shared monolayer aperture l40, l4lc has an optical aperture axis O_a parallel to the height H of the monolithic converters, and has a smallest cross-sectional area Ao perpendicular to the optical aperture axis O_a. The corresponding set l25c of two or more adjacently configured monolithic converters has a largest cross-sectional area Auu in a plane perpendicular to the optical aperture axis O_a, wherein A_a:, ~ 0.03 *A_m3. Note that the hyphenated borders indicate the borders of cross-sectional areas. A fourth set l25d of two or more, especially four, adjacently configured monolithic converters defines a fourth shared monolayer aperture l40,l4ld. Hence, in this embodiment, the fourth shared monolayer aperture 140, 14 ld is defined by the side surfaces of four adjacently configured monolithic converters 120. i.e., the monolithic converters 120 diagonally arranged with respect to the second shared monolayer aperture 140, 14 ld do not border it with a side surface and are therefore not part of the set l25d of two or more adjacently configured monolithic converters. In this embodiment, the smallest cross-sectional area A r of the fourth shared monolayer aperture 140, l4ld perpendicular to an optical aperture axis O_a approximately equals the cross-sectional area of a single monolithic converter 120 perpendicular to the optical aperture axis O_a, i.e., the smallest-cross-sectional area A_a4 ~ Amax (not drawn). Hence, Aa4 = 0.25 *A_m4, wherein A_m4 is the largest cross- sectional area of the fourth set l25d of two or more adjacently configured monolithic converters in a plane perpendicular to the optical aperture axis O_a.

In specific embodiments, the active collimator system comprises a monolayer 105, especially shaped as a square grid, of adjacently configured monolithic converters 120, wherein sets 125 of two or more, especially four, adjacently configured monolithic converters define shared monolayer apertures 140, 141, wherein the cross-sectional area A_a of a shared monolayer aperture is approximately equal to the largest cross-sectional area A_max of an individual monolithic converter of the set 125 of two or more adjacently configured monolayer converters, wherein the active collimator system further comprises a metal collimator 180 comprising a second coded aperture 1 12, wherein the second coded aperture 1 12 comprises a metal collimator aperture, wherein the metal collimator 180 is arranged such that the metal collimator aperture is aligned with the shared monolayer aperture 140, 141 of the set of two or more adjacently configured monolithic converters 120. Especially, A_a < 0.25*A_m.

Fig. 8b schematically depicts a top view of a cross-section of another embodiment comprising shared monolayer apertures l40,l4le,l4lf, and corresponding sets l25e, l25f of two or more adjacently configured monolithic converters. In this embodiment, the monolithic converters 120 have a regular hexagonal prismatic shape. Hence, the monolayer of adjacently configured monolithic converters 120 resembles a grid of hexagons having cells, gridlines and intersections. A third set l25e oftwo or more, especially six, adjacently configured monolithic converters defines a fifth shared monolayer aperture l40,l4le at their shared intersection in the grid. The smallest cross-sectional area _D of the fifth shared monolayer aperture l40, l4le perpendicular to its optical aperture axis O_a is approximately equal to 0. l *A_m5, i.e., A_a5 ~ 0.1 *A_m\ wherein A_ms is the largest cross-sectional area in a plane perpendicular to the optical aperture axis O_a of the fifth set l25e of two or more adjacently configured monolithic converters. Similarly, a sixth set l25f of two or more, especially three, adjacently configured monolithic converters defines a sixth shared monolayer aperture l40, l4lf. The smallest cross-sectional area A_ar, of the sixth shared monolayer aperture l40, l4le perpendicular to its optical aperture axis O_a is approximately equal to 0.33 *A_m6, i.e., A_ae ~ 033* A_me, wherein A,,,,·, is the largest cross-sectional area in a plane perpendicular to the optical aperture axis O_a of the sixth set l25f of two or more adjacently configured monolithic converters. Amongst others, active collimation may allow for one or more imaging modalities to be integrated into a single imaging bore to measure gamma rays emitted from multiple SPECT and PET molecular vectors simultaneously whilst using X-ray/MR functionality, and/or one or more other functionalities, to obtain anatomical information of the subject. Hence, e.g. the nuclear imaging system 1000 may further comprise or be functionally coupled to one or more of a positron emission tomography (PET) imaging system, a single photon emission computed tomography (SPECT) imaging system, a computed tomography (CT) imaging system, and a magnetic resonance imaging (MRI) imaging system... Hence, in embodiments active collimation may be coupled with essentially any imaging modality that can obtain additional anatomical (i.e. ultrasound, tomosynthesis, etc.) or functional (optical fluorescence, magnetic particle) information. Hence, in embodiments, the nuclear imaging may comprise a PET- SPECT detector system, and may further comprise or be functionally coupled to one or more other anatomical and/or functional imaging systems

In embodiments, the active collimator system and the nuclear imaging system as defined herein may be used for treatment delivery monitoring in radiotherapy treatment.

The term“plurality” refers to two or more.

The terms“substantially” or“essentially” herein, and similar terms, will be understood by the person skilled in the art. The terms“substantially” or “essentially” may also include embodiments with“entirely”,“completely”,“all”, etc. Hence, in embodiments the adjective substantially or essentially may also be removed. Where applicable, the term“substantially” or the term“essentially” may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%.

As known to a person skilled in the art, the term“<” may in embodiments refer to“=” and may in other embodiments refer to“<”; likewise the term“>” may in embodiments refer to“=” and may in other embodiments refer to “>”; likewise the term

is known to refer to“approximately equal to”, such as the allowable values selected from the range being <10% (i.e. +/- 10%), especially <5 %, such as <1%. E.g. ~ 100 may refer to 90-110. The term“comprise” includes also embodiments wherein the term “comprises” means“consists of’.

The term“and/or” especially relates to one or more of the items mentioned before and after“and/or”. For instance, a phrase“item 1 and/or item 2” and similar phrases may relate to one or more of item 1 and item 2. The term "comprising" may in an embodiment refer to "consisting of' but may in another embodiment also refer to "containing at least the defined species and optionally one or more other species".

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

The devices, apparatus, or systems may herein amongst others be described during operation. As will be clear to the person skilled in the art, the invention is not limited to methods of operation, or devices, apparatus, or systems in operation.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim.

Use of the verb "to comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. Unless the context clearly requires otherwise, throughout the description and the claims, the words“comprise”,“comprising”, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.

The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In a device claim, or an apparatus claim, or a system claim, enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

The invention also provides a control system that may control the device, apparatus, or system, or that may execute the herein described method or process. Yet further, the invention also provides a computer program product, when arnning on a computer which is functionally coupled to or comprised by the device, apparatus, or system, controls one or more controllable elements of such device, apparatus, or system.

The invention further applies to a device, apparatus, or system comprising one or more of the characterizing features described in the description and/or shown in the attached drawings. The invention further pertains to a method or process comprising one or more of the characterizing features described in the description and/or shown in the attached drawings.

The various aspects discussed in this patent can be combined in order to provide additional advantages. Further, the person skilled in the art will understand that embodiments can be combined, and that also more than two embodiments can be combined. Furthermore, some of the features can form the basis for one or more divisional applications.

Claims

CLAIMS:

1. An active collimator system (100) having a coded aperture (110), the active collimator system (100) comprising (i) a monolayer (105) of adjacently configured monolithic converters (120), wherein the monolithic converters (120) are selected from the group consisting of scintillators and semiconductors, wherein the monolithic converters (120) are configured to convert incident gamma rays (160) and to provide a conversion product (161), and (ii) a sensor element (130) configured to detect the conversion product (161); wherein the sensor element (130) is configured at a surface of one or more of the monolithic converters (120); and wherein a set (125) of two or more adjacently configured monolithic converters (120) define a shared monolayer aperture (140, 141), wherein the coded aperture (110) comprises the shared monolayer aperture (140,141), wherein the shared monolayer aperture (140, 141) has an optical aperture axis O_a, wherein A_:, is a smallest cross-sectional area of the shared monolayer aperture (140,141) perpendicular to the optical aperture axis O_a, and wherein A_m is the largest cross- sectional area of the set (125) of two or more adjacently configured monolithic converters with a plane perpendicular to the optical aperture axis O_a, and wherein Aa < 0.25* A_m.

2. The active collimator system according to claim 1, wherein a set of two adjacently configured monolithic converters defines at least one shared monolayer aperture of the coded aperture.

3. The active collimator system according to claim 2, wherein:

- the at least one shared monolayer aperture of the coded aperture is formed by adjacently configured cavities in both adjacently configured monolithic converters of the set; or

- the at least one shared monolayer aperture of the coded aperture is formed by a single cavity in one of the adjacently configured monolithic converters of the set.

4. The active collimator system according to claim 1, wherein a set of three or more adjacently configured monolithic converters defines at least one shared monolayer aperture of the coded aperture.

5. The active collimator system according to claim 4, wherein the at least one shared monolayer aperture of the coded aperture is formed by adjacently configured cavities in all of the three or more adjacently configured monolithic converters of the set.

6. The active collimator system according to any one of the preceding claims, comprising one or more sets as defined in any one of claims 2-3 and comprising one or more sets as defined in any one of claims 4-5.

7. The active collimator system according to any one of the preceding claims, wherein A_a < 0.2*A_m, and wherein A_a > 0.001 *A_m.

8. The active collimator system according to any one of the preceding claims, wherein at least one of the monolithic converters of a set of two or more adjacently configured monolithic converters comprises a slanted surface, wherein the slanted surface defines at least part of the shared monolayer aperture.

9. The active collimator system according to any one of the preceding claims, wherein the monolayer aperture has a shape selected from the group comprising a rectangular shape, a cylindrical shape, a conical shape, or a pyramidal shape.

10. The active collimator system according to any one of the preceding claims, wherein the coded aperture has one or more of a parallel-hole shape, a pinhole shape, a converging shape, or a diverging shape.

1 1. The active collimator system according to any one of the preceding claims, wherein the monolithic converters of the set of two or more adjacently configured monolithic converters have heights H and have virtual planes (P) perpendicular to the heights H, wherein the virtual planes (P) of the set of two or more adjacently configured monolithic converters are arranged at an angle a, wherein 45° < a < 180°.

12. The active collimator system according to any one of the preceding claims, wherein each monolithic converter unit has a size selected from the range of 2 x 2 x 2 mm - 48 x 48 x 48 mm.

13. The active collimator system according to any one of the preceding claims, wherein the sensor element is configured at a part of the surface of one or more of the monolithic converters.

14. The active collimator system according to any one of the preceding claims, wherein the sensor element is configured in between adjacently configured monolithic converters of the set of two or more adjacently configured monolithic converters.

15. The active collimator system according to any one of the preceding claims, wherein the sensor element is functionally coupled to a plurality of monolithic converters.

16. The active collimator system according to any one of claims 1-13, wherein the active collimator system comprises a plurality of sensor elements, wherein each of the sensor elements is functionally coupled to one monolithic converter, and wherein each of the monolithic converters is functionally coupled to one of the sensor elements.

17. The active collimator system according to any one of the preceding claims, wherein the active collimator system further comprises a metal collimator comprising a second coded aperture, wherein the second coded aperture comprises a metal collimator aperture, wherein the metal collimator is arranged such that the metal collimator aperture is aligned with the shared monolayer aperture of the set of two or more adjacently configured monolithic converters.

18. The active collimator system according to any one of the preceding claims, wherein the active collimator system further comprises a metal insert arranged between two or more adjacent monolithic converters to further define a shared monolayer aperture.

19. The active collimator system according to any one of claims 1-15, wherein the active collimator system exclusively comprises a non-magnetic material.

20. The active collimator system according to any one of the preceding claims, wherein:

- the monolithic converters are scintillators, wherein the scintillators are configured to convert incident gamma rays and to provide scintillation photons, wherein the sensor element comprises a photosensor unit configured to detect the scintillation photons; or

- the monolithic converters are semiconductors, wherein the semiconductors are configured to convert incident gamma rays and to provide an electric current, wherein the sensor element comprises an electrode geometry configured to detect the electric current.

21. The active collimator system according to any one of the preceding claims, wherein the adjacently configured monolithic converters are arranged in a grid having a plurality of rectangular grid cells and grid cell lines, wherein each of the grid cells comprises a single monolithic converter having a cuboid shape, wherein intersecting grid cell lines define an intersection, and wherein a set of four adjacently configured monolithic converters define a shared monolayer aperture at the intersection

22. The active collimator system according any one of the preceding claims, wherein any one of the monolithic converters comprises an interior monolayer aperture of the coded aperture.

23. The active collimator system according to any one of the preceding claims, wherein each monolithic converter has a height H, wherein H is smaller than the longest length of the respective monolithic converter in each dimension perpendicular to H.

24. The active collimator system according to any one of the preceding claims, comprising a plurality of sets of two or more adjacently configured monolithic converters, wherein each set of two or more adjacently configured monolithic converters defines at least one shared monolayer aperture of the coded aperture.

25. The active collimator system according to any one of the preceding claims, wherein, wherein the monolithic converter has a first volume V 1, wherein a largest cuboid fitting within the monolithic converter has a second volume V2, wherein 0.8<V2/Vl<l, wherein the monolithic converter has one or more of (i) a rectangular planar surface including a recess and (ii) a truncated edge.

26. A nuclear imaging system (1000), the nuclear imaging system (1000) comprising the active collimator system (100) according to any one of the preceding claims, the nuclear imaging system (1000) further comprising a gamma ray detector (1040) configured downstream of the active collimator system (100), wherein the gamma ray detector (1040) is configured to detect gamma rays (160) that have passed through the coded aperture (110) of the active collimator system (100).

27. The nuclear imaging system according to claim 26, wherein the nuclear imaging system comprises or is functionally coupled to one or more of a positron emission tomography (PET) imaging system, a single photon emission computed tomography (SPECT) imaging system, a computed tomography (CT) imaging system, and a magnetic resonance imaging (MRI) imaging system.

28. The nuclear imaging system according to claim 27, wherein the nuclear imaging comprises or is functionally coupled to a PET-SPECT detector system, and further comprises one or more other anatomical and/or functional imaging systems.

29. A monolithic converter (120) for use in the active collimator system (100) according to any one of claims 1-25, wherein the monolithic converter (120) is selected from the group consisting of scintillators and semiconductors, wherein the monolithic converter has a first volume VI, wherein a largest cuboid fitting within the monolithic converter has a second volume V2, wherein 0.8<V2/Vl<l, wherein the monolithic converter (120) has a rectangular planar surface including a recess, wherein the rectangular planar surface is a rectangular planar side surface, and wherein the recess ains along a height dimension of the side surface..

30. A method for the determination of an emission location of an incident gamma ray (160) using the active collimator system (100) according to any one of claims 1-25 or the nuclear imaging system according to any one of claims 26-28 or the monolithic converter (120) according to claim 29.

31. The method according to claim 30, wherein the method further comprises monitoring treatment delivery in a radiotherapy treatment.